[InstCombine] (-NSW x) u< x --> x s< 0 (PR39480)

Name: (-x) u< x  -->  x s< 0
%neg_x = sub nsw i8 0, %x ; %x must not be INT_MIN
%r = icmp ult i8 %neg_x, %x
  =>
%r = icmp slt i8 %x, 0

https://rise4fun.com/Alive/zSuf

https://bugs.llvm.org/show_bug.cgi?id=39480

Author: LebedevRI

llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp

@@ -3758,6 +3758,11 @@ Instruction *foldICmpXNegX(ICmpInst &I) {
     NewRHS = Constant::getNullValue(X->getType());
     break;
 
+  case ICmpInst::ICMP_ULT:
+    NewPred = ICmpInst::ICMP_SLT;
+    NewRHS = Constant::getNullValue(X->getType());
+    break;
+
   case ICmpInst::ICMP_EQ:
   case ICmpInst::ICMP_NE:
     NewPred = Pred;

llvm/test/Transforms/InstCombine/cmp-x-vs-neg-x.ll

@@ -91,8 +91,7 @@ define i1 @t5(i8 %x) {
 
 define i1 @t6(i8 %x) {
 ; CHECK-LABEL: @t6(
-; CHECK-NEXT:    [[NEG_X:%.*]] = sub nsw i8 0, [[X:%.*]]
-; CHECK-NEXT:    [[CMP:%.*]] = icmp ult i8 [[NEG_X]], [[X]]
+; CHECK-NEXT:    [[CMP:%.*]] = icmp slt i8 [[X:%.*]], 0
 ; CHECK-NEXT:    ret i1 [[CMP]]
 ;
   %neg_x = sub nsw i8 0, %x