[InstCombine] Fix incorrect zero ext in select of lshr/ashr fold

The -1 constant should be sign extended, not zero extended.

Split out from llvm/llvm-project#80309.

Author: nikic

llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp

@@ -681,11 +681,11 @@ static Value *foldSelectICmpLshrAshr(const ICmpInst *IC, Value *TrueVal,
   Value *X, *Y;
   unsigned Bitwidth = CmpRHS->getType()->getScalarSizeInBits();
   if ((Pred != ICmpInst::ICMP_SGT ||
-       !match(CmpRHS,
-              m_SpecificInt_ICMP(ICmpInst::ICMP_SGE, APInt(Bitwidth, -1)))) &&
+       !match(CmpRHS, m_SpecificInt_ICMP(ICmpInst::ICMP_SGE,
+                                         APInt::getAllOnes(Bitwidth)))) &&
       (Pred != ICmpInst::ICMP_SLT ||
-       !match(CmpRHS,
-              m_SpecificInt_ICMP(ICmpInst::ICMP_SGE, APInt(Bitwidth, 0)))))
+       !match(CmpRHS, m_SpecificInt_ICMP(ICmpInst::ICMP_SGE,
+                                         APInt::getZero(Bitwidth)))))
     return nullptr;
 
   // Canonicalize so that ashr is in FalseVal.

llvm/test/Transforms/InstCombine/ashr-lshr.ll

@@ -63,11 +63,8 @@ define i32 @ashr_lshr2(i32 %x, i32 %y) {
 
 define i128 @ashr_lshr2_i128(i128 %x, i128 %y) {
 ; CHECK-LABEL: @ashr_lshr2_i128(
-; CHECK-NEXT:    [[CMP:%.*]] = icmp sgt i128 [[X:%.*]], 5
-; CHECK-NEXT:    [[L:%.*]] = lshr i128 [[X]], [[Y:%.*]]
-; CHECK-NEXT:    [[R:%.*]] = ashr exact i128 [[X]], [[Y]]
-; CHECK-NEXT:    [[RET:%.*]] = select i1 [[CMP]], i128 [[L]], i128 [[R]]
-; CHECK-NEXT:    ret i128 [[RET]]
+; CHECK-NEXT:    [[CMP1:%.*]] = ashr i128 [[X:%.*]], [[Y:%.*]]
+; CHECK-NEXT:    ret i128 [[CMP1]]
 ;
   %cmp = icmp sgt i128 %x, 5
   %l = lshr i128 %x, %y