[llvm] [AMDGPU] Move WWM register pre-allocation to during regalloc (PR #70618)

[Carl Ritson via llvm-commits]

perlfu wrote:

I realized that we need to explicitly ignore SI_SPILL_S32_TO_VGPR instructions which occur in STRICT_WWM sections otherwise they will unintentionally be assigned WWM registers.
This brings the total test diff on this down massive to primarily wwm-reserved-spill.ll, which shows a change in use of registers because SGPR spill now get first access to VGPRs before WWM pre-allocation runs.

This should now be good to go.

https://github.com/llvm/llvm-project/pull/70618