[X86] EltsFromConsecutiveLoads - only attempt to recognise reverse patterns from all load element patterns (#170852)

Noticed while investigating a regression reported after #168706 - don't
attempt this if any elements are zero/undef - there is a potential issue
with reverse(insert_subvector(undef,load(),0)) patterns - but I haven't
managed to create a fail case.

Author: RKSimon

llvm/lib/Target/X86/X86ISelLowering.cpp

@@ -7571,7 +7571,8 @@ static SDValue EltsFromConsecutiveLoads(EVT VT, ArrayRef<SDValue> Elts,
 
   // REVERSE - attempt to match the loads in reverse and then shuffle back.
   // TODO: Do this for any permute or mismatching element counts.
-  if (Depth == 0 && !ZeroMask && TLI.isTypeLegal(VT) && VT.isVector() &&
+  if (Depth == 0 && ZeroMask.isZero() && UndefMask.isZero() &&
+      TLI.isTypeLegal(VT) && VT.isVector() &&
       NumElems == VT.getVectorNumElements()) {
     SmallVector<SDValue, 16> ReverseElts(Elts.rbegin(), Elts.rend());
     if (SDValue RevLd = EltsFromConsecutiveLoads(