ComputeMaskedBits: sub falls through to add, and sub doesn't have the same overflow semantics as add.

Should fix the selfhost failures that started with r127463.

llvm-svn: 127465

llvm/lib/Analysis/ValueTracking.cpp

@@ -431,7 +431,8 @@ void llvm::ComputeMaskedBits(Value *V, const APInt &Mask,
     }
 
     // Are we still trying to solve for the sign bit?
-    if (Mask.isNegative() && !KnownZero.isNegative() && !KnownOne.isNegative()){
+    if (I->getOpcode() == Instruction::Add &&
+        Mask.isNegative() && !KnownZero.isNegative() && !KnownOne.isNegative()){
       OverflowingBinaryOperator *OBO = cast<OverflowingBinaryOperator>(I);
       if (OBO->hasNoSignedWrap()) {
         // Adding two positive numbers can't wrap into negative ...

llvm/test/Transforms/InstCombine/sext.ll

@@ -126,3 +126,13 @@ define void @test11(<2 x i16> %srcA, <2 x i16> %srcB, <2 x i16>* %dst) {
 ; CHECK-NEXT: store <2 x i16>
 ; CHECK-NEXT: ret
 }                                                                                                                               
+
+define i64 @test12(i32 %x) nounwind {
+  %shr = lshr i32 %x, 1
+  %sub = sub nsw i32 0, %shr
+  %conv = sext i32 %sub to i64
+  ret i64 %conv
+; CHECK: @test12
+; CHECK: sext
+; CHECK: ret
+}