Move ashr optimization from InstCombineShift to InstSimplify.

Refactor code, no functionality change, test case moved from instcombine to instsimplify.

Differential Revision: http://reviews.llvm.org/D4102
 


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213231 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Analysis/InstructionSimplify.cpp

@@ -1346,6 +1346,11 @@ static Value *SimplifyAShrInst(Value *Op0, Value *Op1, bool isExact,
       cast<OverflowingBinaryOperator>(Op0)->hasNoSignedWrap())
     return X;
 
+  // Arithmetic shifting an all-sign-bit value is a no-op.
+  unsigned NumSignBits = ComputeNumSignBits(Op0);
+  if (NumSignBits == Op0->getType()->getScalarSizeInBits())
+    return Op0;
+
   return nullptr;
 }