[llvm] [AMDGPU] Add option to pre-allocate SGPR spill VGPRs (PR #70626)

[via llvm-commits]

ruiling wrote:

I think it is reasonable to let `SIPreAllocateWWMRegs` allocate wwm register given that wwm register allocation by RA is pretty broken. Maybe it is better to make this default behavior. The trick we have done in ` SILowerSGPRSpills::extendWWMVirtRegLiveness()` no longer helps correctness when wwm register is either spilled or its live-range was split. So, can we teach RA to neither spill virtual WWM registers nor split its live-range?


https://github.com/llvm/llvm-project/pull/70626