[X86] getTargetConstantBitsFromNode - don't support partial undefs by default.

Noticed by inspection - as undef bits are seen as zero, we could be matching in places where we require a zero but end up referencing an undef that could then get changed in a later fold.

Creating a test case has proven difficult as multi-use limits usually saves us, but really we shouldn't be allowing partial undefs for anything but constant folding.

Author: RKSimon

llvm/lib/Target/X86/X86ISelLowering.cpp

@@ -4596,7 +4596,7 @@ static bool getTargetConstantBitsFromNode(SDValue Op, unsigned EltSizeInBits,
                                           APInt &UndefElts,
                                           SmallVectorImpl<APInt> &EltBits,
                                           bool AllowWholeUndefs = true,
-                                          bool AllowPartialUndefs = true) {
+                                          bool AllowPartialUndefs = false) {
   assert(EltBits.empty() && "Expected an empty EltBits vector");
 
   Op = peekThroughBitcasts(Op);