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

[Carl Ritson via llvm-commits]

perlfu wrote:

This is based on #70618 - but I have not included those changes in this PR for clarify.
Note: this is not the only way to a achieve this, we could modify the SGPR spill pass to force phys allocation with a similar amount of code diff.

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