[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 https://github.com/llvm/llvm-project/issues/94761.