[X86] lowerShuffleAsLanePermuteAndPermute - ensure we've simplified the demanded shuffle mask elts before testing for a matching shuffle (#153554)

When lowering using sublane shuffles, we can sometimes end up with the
same mask as we started with. We already bail in these occasions, but we
weren't fully simplifying the new shuffle mask before testing if it
matched.

Fixes #153457

Author: RKSimon

llvm/lib/Target/X86/X86ISelLowering.cpp

@@ -15419,18 +15419,18 @@ static SDValue lowerShuffleAsLanePermuteAndPermute(
         return SDValue();
     }
 
-    // Avoid returning the same shuffle operation. For example,
-    // t7: v16i16 = vector_shuffle<8,9,10,11,4,5,6,7,0,1,2,3,12,13,14,15> t5,
-    //                             undef:v16i16
-    if (CrossLaneMask == Mask || InLaneMask == Mask)
-      return SDValue();
-
     // Simplify CrossLaneMask based on the actual demanded elements.
     if (V1.hasOneUse())
       for (int i = 0; i != NumElts; ++i)
         if (!DemandedCrossLane[i])
           CrossLaneMask[i] = SM_SentinelUndef;
 
+    // Avoid returning the same shuffle operation. For example,
+    // t7: v16i16 = vector_shuffle<8,9,10,11,4,5,6,7,0,1,2,3,12,13,14,15> t5,
+    //                             undef:v16i16
+    if (CrossLaneMask == Mask || InLaneMask == Mask)
+      return SDValue();
+
     SDValue CrossLane = DAG.getVectorShuffle(VT, DL, V1, V2, CrossLaneMask);
     return DAG.getVectorShuffle(VT, DL, CrossLane, DAG.getUNDEF(VT),
                                 InLaneMask);