[InstCombine] Drop Range attribute when simplifying 'fshl' based on demanded bits

When simplifying operands based on demanded bits, the return value range
of llvm.fshl might change. Keeping the Range attribute might cause
llvm.fshl to generate a poison and lead to miscompile (#124387). Drop the Range
attribute similar to `dropPosonGeneratingFlags` elsewhere.

Author: MaskRay

llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp

@@ -1039,11 +1039,14 @@ Value *InstCombinerImpl::SimplifyDemandedUseBits(Instruction *I,
         APInt DemandedMaskLHS(DemandedMask.lshr(ShiftAmt));
         APInt DemandedMaskRHS(DemandedMask.shl(BitWidth - ShiftAmt));
         if (I->getOperand(0) != I->getOperand(1)) {
-          if (SimplifyDemandedBits(I, 0, DemandedMaskLHS, LHSKnown,
-                                   Depth + 1, Q) ||
+          if (SimplifyDemandedBits(I, 0, DemandedMaskLHS, LHSKnown, Depth + 1,
+                                   Q) ||
               SimplifyDemandedBits(I, 1, DemandedMaskRHS, RHSKnown, Depth + 1,
-                                   Q))
+                                   Q)) {
+            // Range attribute may not longer hold.
+            I->dropPoisonGeneratingReturnAttributes();
             return I;
+          }
         } else { // fshl is a rotate
           // Avoid converting rotate into funnel shift.
           // Only simplify if one operand is constant.

llvm/test/Transforms/InstCombine/fsh.ll

@@ -1069,11 +1069,12 @@ entry:
   ret <2 x i31>  %res
 }
 
+;; #124387 Range attribute no longer holds after operands changed.
 define i8 @fshl_range_trunc(i1 %x) {
 ; CHECK-LABEL: @fshl_range_trunc(
 ; CHECK-NEXT:    [[ZEXT:%.*]] = zext i1 [[X:%.*]] to i32
 ; CHECK-NEXT:    [[OR:%.*]] = or disjoint i32 [[ZEXT]], 126
-; CHECK-NEXT:    [[FSHL:%.*]] = call range(i32 -4, 2) i32 @llvm.fshl.i32(i32 [[OR]], i32 -2, i32 1)
+; CHECK-NEXT:    [[FSHL:%.*]] = call i32 @llvm.fshl.i32(i32 [[OR]], i32 -2, i32 1)
 ; CHECK-NEXT:    [[TR:%.*]] = trunc nuw i32 [[FSHL]] to i8
 ; CHECK-NEXT:    ret i8 [[TR]]
 ;