[SimplifyLibCalls] Fix memchr misoptimization (#106121)

The `ch` argument of memcmp should be truncated to `unsigned char`
before using it in comparisons. This didn't happen on all code paths.
The following program miscompiled at -O1 and higher:

```C++
#include <cstring>
#include <iostream>

char ch = '\x81';

int main() {
    bool found = std::strchr("\x80\x81\x82", ch) != nullptr;
    std::cout << std::boolalpha << found << '\n';
}
```

Author: s-barannikov

llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp

@@ -1454,10 +1454,12 @@ Value *LibCallSimplifier::optimizeMemChr(CallInst *CI, IRBuilderBase &B) {
     if (NonContRanges > 2)
       return nullptr;
 
+    // Slice off the character's high end bits.
+    CharVal = B.CreateTrunc(CharVal, B.getInt8Ty());
+
     SmallVector<Value *> CharCompares;
     for (unsigned char C : SortedStr)
-      CharCompares.push_back(
-          B.CreateICmpEQ(CharVal, ConstantInt::get(CharVal->getType(), C)));
+      CharCompares.push_back(B.CreateICmpEQ(CharVal, B.getInt8(C)));
 
     return B.CreateIntToPtr(B.CreateOr(CharCompares), CI->getType());
   }

llvm/test/Transforms/InstCombine/memchr-7.ll

@@ -12,11 +12,12 @@ declare ptr @memchr(ptr, i32, i64)
 
 define zeroext i1 @strchr_to_memchr_n_equals_len(i32 %c) {
 ; CHECK-LABEL: @strchr_to_memchr_n_equals_len(
-; CHECK-NEXT:    [[TMP1:%.*]] = icmp eq i32 [[C:%.*]], 0
-; CHECK-NEXT:    [[TMP2:%.*]] = add i32 [[C]], -97
-; CHECK-NEXT:    [[TMP3:%.*]] = icmp ult i32 [[TMP2]], 26
-; CHECK-NEXT:    [[TMP4:%.*]] = or i1 [[TMP1]], [[TMP3]]
-; CHECK-NEXT:    ret i1 [[TMP4]]
+; CHECK-NEXT:    [[TMP1:%.*]] = trunc i32 [[C:%.*]] to i8
+; CHECK-NEXT:    [[TMP2:%.*]] = icmp eq i8 [[TMP1]], 0
+; CHECK-NEXT:    [[TMP3:%.*]] = add i8 [[TMP1]], -97
+; CHECK-NEXT:    [[TMP4:%.*]] = icmp ult i8 [[TMP3]], 26
+; CHECK-NEXT:    [[TMP5:%.*]] = or i1 [[TMP2]], [[TMP4]]
+; CHECK-NEXT:    ret i1 [[TMP5]]
 ;
   %call = tail call ptr @strchr(ptr nonnull dereferenceable(27) @.str, i32 %c)
   %cmp = icmp ne ptr %call, null
@@ -38,9 +39,10 @@ define zeroext i1 @memchr_n_equals_len(i32 %c) {
 
 define zeroext i1 @memchr_n_less_than_len(i32 %c) {
 ; CHECK-LABEL: @memchr_n_less_than_len(
-; CHECK-NEXT:    [[TMP1:%.*]] = add i32 [[C:%.*]], -97
-; CHECK-NEXT:    [[TMP2:%.*]] = icmp ult i32 [[TMP1]], 15
-; CHECK-NEXT:    ret i1 [[TMP2]]
+; CHECK-NEXT:    [[TMP1:%.*]] = trunc i32 [[C:%.*]] to i8
+; CHECK-NEXT:    [[TMP2:%.*]] = add i8 [[TMP1]], -97
+; CHECK-NEXT:    [[TMP3:%.*]] = icmp ult i8 [[TMP2]], 15
+; CHECK-NEXT:    ret i1 [[TMP3]]
 ;
   %call = tail call ptr @memchr(ptr @.str, i32 %c, i64 15)
   %cmp = icmp ne ptr %call, null
@@ -50,11 +52,12 @@ define zeroext i1 @memchr_n_less_than_len(i32 %c) {
 
 define zeroext i1 @memchr_n_more_than_len(i32 %c) {
 ; CHECK-LABEL: @memchr_n_more_than_len(
-; CHECK-NEXT:    [[TMP1:%.*]] = icmp eq i32 [[C:%.*]], 0
-; CHECK-NEXT:    [[TMP2:%.*]] = add i32 [[C]], -97
-; CHECK-NEXT:    [[TMP3:%.*]] = icmp ult i32 [[TMP2]], 26
-; CHECK-NEXT:    [[TMP4:%.*]] = or i1 [[TMP1]], [[TMP3]]
-; CHECK-NEXT:    ret i1 [[TMP4]]
+; CHECK-NEXT:    [[TMP1:%.*]] = trunc i32 [[C:%.*]] to i8
+; CHECK-NEXT:    [[TMP2:%.*]] = icmp eq i8 [[TMP1]], 0
+; CHECK-NEXT:    [[TMP3:%.*]] = add i8 [[TMP1]], -97
+; CHECK-NEXT:    [[TMP4:%.*]] = icmp ult i8 [[TMP3]], 26
+; CHECK-NEXT:    [[TMP5:%.*]] = or i1 [[TMP2]], [[TMP4]]
+; CHECK-NEXT:    ret i1 [[TMP5]]
 ;
   %call = tail call ptr @memchr(ptr @.str, i32 %c, i64 30)
   %cmp = icmp ne ptr %call, null
@@ -114,12 +117,13 @@ define zeroext i1 @memchr_n_equals_len2_minsize(i32 %c) minsize {
 ; Positive test - 2 non-contiguous ranges
 define zeroext i1 @strchr_to_memchr_2_non_cont_ranges(i32 %c) {
 ; CHECK-LABEL: @strchr_to_memchr_2_non_cont_ranges(
-; CHECK-NEXT:    [[TMP1:%.*]] = add i32 [[C:%.*]], -97
-; CHECK-NEXT:    [[TMP2:%.*]] = icmp ult i32 [[TMP1]], 6
-; CHECK-NEXT:    [[TMP3:%.*]] = add i32 [[C]], -109
-; CHECK-NEXT:    [[TMP4:%.*]] = icmp ult i32 [[TMP3]], 3
-; CHECK-NEXT:    [[TMP5:%.*]] = or i1 [[TMP2]], [[TMP4]]
-; CHECK-NEXT:    ret i1 [[TMP5]]
+; CHECK-NEXT:    [[TMP1:%.*]] = trunc i32 [[C:%.*]] to i8
+; CHECK-NEXT:    [[TMP2:%.*]] = add i8 [[TMP1]], -97
+; CHECK-NEXT:    [[TMP3:%.*]] = icmp ult i8 [[TMP2]], 6
+; CHECK-NEXT:    [[TMP4:%.*]] = add i8 [[TMP1]], -109
+; CHECK-NEXT:    [[TMP5:%.*]] = icmp ult i8 [[TMP4]], 3
+; CHECK-NEXT:    [[TMP6:%.*]] = or i1 [[TMP3]], [[TMP5]]
+; CHECK-NEXT:    ret i1 [[TMP6]]
 ;
   %call = tail call ptr @memchr(ptr @.str.2, i32 %c, i64 9)
   %cmp = icmp ne ptr %call, null
@@ -129,12 +133,13 @@ define zeroext i1 @strchr_to_memchr_2_non_cont_ranges(i32 %c) {
 ; Positive test - 2 non-contiguous ranges with char duplication
 define zeroext i1 @strchr_to_memchr_2_non_cont_ranges_char_dup(i32 %c) {
 ; CHECK-LABEL: @strchr_to_memchr_2_non_cont_ranges_char_dup(
-; CHECK-NEXT:    [[TMP1:%.*]] = add i32 [[C:%.*]], -97
-; CHECK-NEXT:    [[TMP2:%.*]] = icmp ult i32 [[TMP1]], 3
-; CHECK-NEXT:    [[TMP3:%.*]] = add i32 [[C]], -109
-; CHECK-NEXT:    [[TMP4:%.*]] = icmp ult i32 [[TMP3]], 2
-; CHECK-NEXT:    [[TMP5:%.*]] = or i1 [[TMP2]], [[TMP4]]
-; CHECK-NEXT:    ret i1 [[TMP5]]
+; CHECK-NEXT:    [[TMP1:%.*]] = trunc i32 [[C:%.*]] to i8
+; CHECK-NEXT:    [[TMP2:%.*]] = add i8 [[TMP1]], -97
+; CHECK-NEXT:    [[TMP3:%.*]] = icmp ult i8 [[TMP2]], 3
+; CHECK-NEXT:    [[TMP4:%.*]] = add i8 [[TMP1]], -109
+; CHECK-NEXT:    [[TMP5:%.*]] = icmp ult i8 [[TMP4]], 2
+; CHECK-NEXT:    [[TMP6:%.*]] = or i1 [[TMP3]], [[TMP5]]
+; CHECK-NEXT:    ret i1 [[TMP6]]
 ;
   %call = tail call ptr @memchr(ptr @.str.4, i32 %c, i64 6)
   %cmp = icmp ne ptr %call, null