[InstCombine] fold icmp of offset value with constant

There must be a better way to describe this pattern in words?
(X + C2) >u C --> X <s -C2 (if C == C2 + SMAX)

This could be extended to handle the more general (non-constant)
pattern too:
https://alive2.llvm.org/ce/z/rdfNFP

  define i1 @src(i8 %a, i8 %c1) {
    %t = add i8 %a, %c1
    %c2 = add i8 %c1, 127 ; SMAX
    %ov = icmp ugt i8 %t, %c2
    ret i1 %ov
  }

  define i1 @tgt(i8 %a, i8 %c1) {
    %neg_c1 = sub i8 0, %c1
    %ov = icmp slt i8 %a, %neg_c1
    ret i1 %ov
  }

The pattern was noticed as a by-product of D104932.

Author: rotateright

llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp

@@ -2638,6 +2638,13 @@ Instruction *InstCombinerImpl::foldICmpAddConstant(ICmpInst &Cmp,
   Type *Ty = Add->getType();
   CmpInst::Predicate Pred = Cmp.getPredicate();
 
+  // Fold an unsigned compare with offset to signed compare:
+  // (X + C2) >u C --> X <s -C2 (if C == C2 + SMAX)
+  // TODO: Find the ULT and signed predicate siblings.
+  if (Pred == CmpInst::ICMP_UGT &&
+      C == *C2 + APInt::getSignedMaxValue(Ty->getScalarSizeInBits()))
+    return new ICmpInst(ICmpInst::ICMP_SLT, X, ConstantInt::get(Ty, -(*C2)));
+
   // If the add does not wrap, we can always adjust the compare by subtracting
   // the constants. Equality comparisons are handled elsewhere. SGE/SLE/UGE/ULE
   // are canonicalized to SGT/SLT/UGT/ULT.

llvm/test/Transforms/InstCombine/icmp-add.ll

@@ -750,8 +750,7 @@ define i1 @with_nuw_large_negative(i8 %x, i8 %y) {
 
 define i1 @ugt_offset(i8 %a) {
 ; CHECK-LABEL: @ugt_offset(
-; CHECK-NEXT:    [[T:%.*]] = add i8 [[A:%.*]], 124
-; CHECK-NEXT:    [[OV:%.*]] = icmp ugt i8 [[T]], -5
+; CHECK-NEXT:    [[OV:%.*]] = icmp slt i8 [[A:%.*]], -124
 ; CHECK-NEXT:    ret i1 [[OV]]
 ;
   %t = add i8 %a, 124
@@ -763,7 +762,7 @@ define i1 @ugt_offset_use(i32 %a) {
 ; CHECK-LABEL: @ugt_offset_use(
 ; CHECK-NEXT:    [[T:%.*]] = add i32 [[A:%.*]], 42
 ; CHECK-NEXT:    call void @use(i32 [[T]])
-; CHECK-NEXT:    [[OV:%.*]] = icmp ugt i32 [[T]], -2147483607
+; CHECK-NEXT:    [[OV:%.*]] = icmp slt i32 [[A]], -42
 ; CHECK-NEXT:    ret i1 [[OV]]
 ;
   %t = add i32 %a, 42
@@ -774,15 +773,16 @@ define i1 @ugt_offset_use(i32 %a) {
 
 define <2 x i1> @ugt_offset_splat(<2 x i5> %a) {
 ; CHECK-LABEL: @ugt_offset_splat(
-; CHECK-NEXT:    [[T:%.*]] = add <2 x i5> [[A:%.*]], <i5 9, i5 9>
-; CHECK-NEXT:    [[OV:%.*]] = icmp ugt <2 x i5> [[T]], <i5 -8, i5 -8>
+; CHECK-NEXT:    [[OV:%.*]] = icmp slt <2 x i5> [[A:%.*]], <i5 -9, i5 -9>
 ; CHECK-NEXT:    ret <2 x i1> [[OV]]
 ;
   %t = add <2 x i5> %a, <i5 9, i5 9>
   %ov = icmp ugt <2 x i5> %t, <i5 24, i5 24>
   ret <2 x i1> %ov
 }
 
+; negative test - constants must differ by SMAX
+
 define i1 @ugt_wrong_offset(i8 %a) {
 ; CHECK-LABEL: @ugt_wrong_offset(
 ; CHECK-NEXT:    [[T:%.*]] = add i8 [[A:%.*]], 123