[AArch64] Allow sinking both extract and splat to smull

We currently will sink either a splat or two extracts to a smull/umull.
There can be cases where we have both one operand as an extract and one
as a splat. This alters the areExtractShuffleVectors method to account
for that case.

Differential Revision: https://reviews.llvm.org/D137726