[AArch64] match splat of bitcasted extract subvector to DUPLANE

This is another potential regression exposed by D63815.

Here we peek through a bitcast to find an extract subvector and
scale the splat offset based on that:
splat (bitcast (extract X, C)), LaneC --> duplane (bitcast X), LaneC'

Differential Revision: https://reviews.llvm.org/D71672