[X86] getSHUFPDImm - allow general shuffle mask size

#116419 assumed that getSHUFPDImm incorrectly hardcoded the mask size to 4 (cut+pasta typo from getV4X86ShuffleImm).

Waiting on reduced test case from @metaflow

Author: RKSimon

llvm/lib/Target/X86/X86ISelLowering.cpp

@@ -9937,7 +9937,8 @@ static unsigned getSHUFPDImm(ArrayRef<int> Mask) {
   // If the mask only uses one non-undef element, then fully 'splat' it to
   // improve later broadcast matching.
   int FirstIndex = find_if(Mask, [](int M) { return M >= 0; }) - Mask.begin();
-  assert(0 <= FirstIndex && FirstIndex < 4 && "All undef shuffle mask");
+  assert(0 <= FirstIndex && FirstIndex < Mask.size() &&
+         "All undef shuffle mask");
 
   int FirstElt = Mask[FirstIndex];
   if (all_of(Mask, [FirstElt](int M) { return M < 0 || M == FirstElt; }) &&