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

[Matt Arsenault via llvm-commits]

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@@ -201,6 +205,10 @@ bool SIPreAllocateWWMRegs::runOnMachineFunction(MachineFunction &MF) {
 
   RegClassInfo.runOnMachineFunction(MF);
 
+  bool PreallocateSGPRSpillVGPRs =
+      EnablePreallocateSGPRSpillVGPRs ||
+      MF.getFunction().hasFnAttribute("amdgpu-prealloc-sgpr-spill-vgprs");
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arsenm wrote:

Don't see why we would want this controllable by an attribute for migration purposes a flag is sufficient.

@cdevadas is also working on getting the proper allocator to handle the allocation of WWM registers 

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