[DAG] Add a generic expansion for SHIFT_PARTS opcodes using funnel shifts

[DAG] Add a generic expansion for SHIFT_PARTS opcodes using funnel shifts

Based off a discussion on D89281 - where the AARCH64 implementations were being replaced to use funnel shifts.

Any target that has efficient funnel shift lowering can handle the shift parts expansion using the same expansion, avoiding a lot of duplication.

I've generalized the X86 implementation and moved it to TargetLowering - so far I've found that AARCH64 and AMDGPU benefit, but many other targets (ARM, PowerPC + RISCV in particular) could easily use this with a few minor improvements to their funnel shift lowering (or the folding of their target ops that funnel shifts lower to).

NOTE: I'm trying to avoid adding full SHIFT_PARTS legalizer handling as I think it might actually be possible to remove these opcodes in the medium-term and use funnel shift / libcall expansion directly.

Differential Revision: https://reviews.llvm.org/D101987