[AArch64] Ensure we do not access illegal operands in tryCombineMULLWithUZP1

https://github.com/llvm/llvm-project/issues/65015 shows a case where
tryCombineMULLWithUZP1 could attempt to look at the wrong operand of another
user instruction. This adds an extra else as if we don't find the right opcode,
we don't need to check the operands.

Differential Revision: https://reviews.llvm.org/D159282