[llvm] [NFC][AMDGPU] Compute always reserved registers once (PR #132006)

[Shilei Tian via llvm-commits]

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@@ -629,11 +629,16 @@ SIRegisterInfo::getMaxNumVectorRegs(const MachineFunction &MF) const {
   return std::pair(MaxNumVGPRs, MaxNumAGPRs);
 }
 
-BitVector SIRegisterInfo::getReservedRegs(const MachineFunction &MF) const {
-  BitVector Reserved(getNumRegs());
-  Reserved.set(AMDGPU::MODE);
+BitVector SIRegisterInfo::AlwaysReservedRegs;
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shiltian wrote:

You can just move this to `getAlwaysReservedRegs` and make it static there.

https://github.com/llvm/llvm-project/pull/132006