[X86] getFauxShuffleMask - only handle VTRUNC nodes with matching src/dst sizes (#134161)

Cleanup work for #133947 - we need to handle VTRUNC nodes with large
source vectors directly to allow us to widen the size of the shuffle
combine

We currently discard these results in combineX86ShufflesRecursively
anyhow as we don't allow inputs from getTargetShuffleInputs to be larger
than the shuffle value type

Author: RKSimon

llvm/lib/Target/X86/X86ISelLowering.cpp

@@ -6413,9 +6413,7 @@ static bool getFauxShuffleMask(SDValue N, const APInt &DemandedElts,
   case X86ISD::VTRUNC: {
     SDValue Src = N.getOperand(0);
     EVT SrcVT = Src.getValueType();
-    // Truncated source must be a simple vector.
-    if (!SrcVT.isSimple() || (SrcVT.getSizeInBits() % 128) != 0 ||
-        (SrcVT.getScalarSizeInBits() % 8) != 0)
+    if (SrcVT.getSizeInBits() != NumSizeInBits)
       return false;
     unsigned NumSrcElts = SrcVT.getVectorNumElements();
     unsigned NumBitsPerSrcElt = SrcVT.getScalarSizeInBits();