[VectorCombine] transform bitcasted shuffle to wider elements

bitcast (shuf V, MaskC) --> shuf (bitcast V), MaskC'

This is the widen shuffle elements enhancement to D76727.
It builds on the analysis and simplifications in
D77881 and rG6a7e958a423e.

The phase ordering tests show that we can simplify inverse
shuffles across a binop in both directions (widen/narrow or
narrow/widen) now.

There's another potential transform visible in some of the
remaining TODOs - move a bitcasted operand of a shuffle
after the shuffle.

Differential Revision: https://reviews.llvm.org/D78371