[InstCombine] Require immediate constant in canEvaluateShifted()

Otherwise we risk infinite loops when shift constant expressions
are no longer supported.

Author: nikic

llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp

@@ -560,8 +560,8 @@ static bool canEvaluateShiftedShift(unsigned OuterShAmt, bool IsOuterShl,
 /// this succeeds, getShiftedValue() will be called to produce the value.
 static bool canEvaluateShifted(Value *V, unsigned NumBits, bool IsLeftShift,
                                InstCombinerImpl &IC, Instruction *CxtI) {
-  // We can always evaluate constants shifted.
-  if (isa<Constant>(V))
+  // We can always evaluate immediate constants.
+  if (match(V, m_ImmConstant()))
     return true;
 
   Instruction *I = dyn_cast<Instruction>(V);