[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