[llvm] [AMDGPU] SILowerSGPRSpills: do not update MRI reserve registers (PR #77888)

[Christudasan Devadasan via llvm-commits]

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@@ -442,12 +440,5 @@ bool SILowerSGPRSpills::runOnMachineFunction(MachineFunction &MF) {
   SaveBlocks.clear();
   RestoreBlocks.clear();
 
-  // Updated the reserved registers with any physical VGPRs added for SGPR
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cdevadas wrote:

The phys VGPRs used for CSR SGPR spilling shouldn't be reused during SIPreAllocateWWMRegs.
I hope they will be marked reserved while getting the AllocationOrder from `RegClassInfo.runOnMachineFunction(MF)` while computing the AllocationOrder.
@arsenm any specific reason they are marked Reserved here explicitly?

https://github.com/llvm/llvm-project/pull/77888