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

[via llvm-commits]

ruiling wrote:

> You may need to check within processDef() whether a virtual register was already pushed into `RegsToRewrite` so we don't need to process one register several times as sgpr spill may have lots of re-definition. Other parts LGTM.

A second look, `VRM->hasPhys(Reg)` should already do that?

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