[InstCombine] canNarrowShiftAmt - replace custom Constant matching with m_SpecificInt_ICMP

The existing code ignores undef values which matches m_SpecificInt_ICMP, although m_SpecificInt_ICMP returns false for an all-undef constant, I've added test coverage at rGfe0197e194a64f9 to show that undef folding should already have dealt with that case.

Author: RKSimon

llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp

@@ -1701,27 +1701,8 @@ static Instruction *foldOrToXor(BinaryOperator &I,
 /// Return true if a constant shift amount is always less than the specified
 /// bit-width. If not, the shift could create poison in the narrower type.
 static bool canNarrowShiftAmt(Constant *C, unsigned BitWidth) {
-  if (auto *ScalarC = dyn_cast<ConstantInt>(C))
-    return ScalarC->getZExtValue() < BitWidth;
-
-  if (C->getType()->isVectorTy()) {
-    // Check each element of a constant vector.
-    unsigned NumElts = cast<FixedVectorType>(C->getType())->getNumElements();
-    for (unsigned i = 0; i != NumElts; ++i) {
-      Constant *Elt = C->getAggregateElement(i);
-      if (!Elt)
-        return false;
-      if (isa<UndefValue>(Elt))
-        continue;
-      auto *CI = dyn_cast<ConstantInt>(Elt);
-      if (!CI || CI->getZExtValue() >= BitWidth)
-        return false;
-    }
-    return true;
-  }
-
-  // The constant is a constant expression or unknown.
-  return false;
+  APInt Threshold(C->getType()->getScalarSizeInBits(), BitWidth);
+  return match(C, m_SpecificInt_ICMP(ICmpInst::ICMP_ULT, Threshold));
 }
 
 /// Try to use narrower ops (sink zext ops) for an 'and' with binop operand and