[AArch64] Fix copy and paste error in tryCombineMULLWithUZP1() (NFCI) (#97729)

The bitcast check was performed on the wrong value in one of the
branches.

I believe this doesn't actually result in any behavior difference,
because the following code looking at ExtractHigh users currently
doesn't try to look through BITCASTS. I've left a TODO for that.

Fixes #94761.

Author: nikic

llvm/lib/Target/AArch64/AArch64ISelLowering.cpp

@@ -24833,7 +24833,7 @@ static SDValue tryCombineMULLWithUZP1(SDNode *N,
   } else if (isEssentiallyExtractHighSubvector(RHS) &&
              LHS.getOpcode() == ISD::TRUNCATE) {
     TruncHigh = LHS;
-    if (LHS.getOpcode() == ISD::BITCAST)
+    if (RHS.getOpcode() == ISD::BITCAST)
       ExtractHigh = RHS.getOperand(0);
     else
       ExtractHigh = RHS;
@@ -24862,6 +24862,7 @@ static SDValue tryCombineMULLWithUZP1(SDNode *N,
   // This dagcombine assumes the two extract_high uses same source vector in
   // order to detect the pair of the mull. If they have different source vector,
   // this code will not work.
+  // TODO: Should also try to look through a bitcast.
   bool HasFoundMULLow = true;
   SDValue ExtractHighSrcVec = ExtractHigh.getOperand(0);
   if (ExtractHighSrcVec->use_size() != 2)