[InstSimplify] Use weak symbol in test to show miscompile (NFC)

This fold is incorrect, because it assumes that all indices are
non-zero. This happens to be true for the test as written, but
doesn't hold if we use an extern weak global instead, for which
ptrtoint might be zero.

Add separate tests for the simple constant int case.

Author: nikic

llvm/test/Transforms/InstSimplify/ConstProp/icmp-global.ll

@@ -65,6 +65,7 @@ define i1 @ult_constexpr_constexpr_one(i8* %x) {
 
 @g = global [2 x i32] [i32 1, i32 2]
 @g2 = global i32 0
+@g2_weak = extern_weak global i32
 
 define i1 @global_ne_null() {
 ; CHECK-LABEL: @global_ne_null(
@@ -121,7 +122,7 @@ define i1 @null_gep_ne_null() {
 ; CHECK-LABEL: @null_gep_ne_null(
 ; CHECK-NEXT:    ret i1 true
 ;
-  %gep = getelementptr i8, i8* null, i64 ptrtoint (i32* @g2 to i64)
+  %gep = getelementptr i8, i8* null, i64 ptrtoint (i32* @g2_weak to i64)
   %cmp = icmp ne i8* %gep, null
   ret i1 %cmp
 }
@@ -130,20 +131,38 @@ define i1 @null_gep_ugt_null() {
 ; CHECK-LABEL: @null_gep_ugt_null(
 ; CHECK-NEXT:    ret i1 true
 ;
-  %gep = getelementptr i8, i8* null, i64 ptrtoint (i32* @g2 to i64)
+  %gep = getelementptr i8, i8* null, i64 ptrtoint (i32* @g2_weak to i64)
   %cmp = icmp ugt i8* %gep, null
   ret i1 %cmp
 }
 
 define i1 @null_gep_sgt_null() {
 ; CHECK-LABEL: @null_gep_sgt_null(
-; CHECK-NEXT:    ret i1 icmp sgt (i8* getelementptr (i8, i8* null, i64 ptrtoint (i32* @g2 to i64)), i8* null)
+; CHECK-NEXT:    ret i1 icmp sgt (i8* getelementptr (i8, i8* null, i64 ptrtoint (i32* @g2_weak to i64)), i8* null)
 ;
-  %gep = getelementptr i8, i8* null, i64 ptrtoint (i32* @g2 to i64)
+  %gep = getelementptr i8, i8* null, i64 ptrtoint (i32* @g2_weak to i64)
   %cmp = icmp sgt i8* %gep, null
   ret i1 %cmp
 }
 
+define i1 @null_gep_ne_null_constant_int() {
+; CHECK-LABEL: @null_gep_ne_null_constant_int(
+; CHECK-NEXT:    ret i1 true
+;
+  %gep = getelementptr i8, i8* null, i64 1
+  %cmp = icmp ne i8* %gep, null
+  ret i1 %cmp
+}
+
+define i1 @null_gep_ugt_null_constant_int() {
+; CHECK-LABEL: @null_gep_ugt_null_constant_int(
+; CHECK-NEXT:    ret i1 true
+;
+  %gep = getelementptr i8, i8* null, i64 1
+  %cmp = icmp ugt i8* %gep, null
+  ret i1 %cmp
+}
+
 define i1 @null_gep_ne_global() {
 ; CHECK-LABEL: @null_gep_ne_global(
 ; CHECK-NEXT:    ret i1 true