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

[Shilei Tian via llvm-commits]

================
@@ -628,12 +628,17 @@ SIRegisterInfo::getMaxNumVectorRegs(const MachineFunction &MF) const {
   return std::pair(MaxNumVGPRs, MaxNumAGPRs);
 }
 
-BitVector SIRegisterInfo::getReservedRegs(const MachineFunction &MF) const {
+BitVector SIRegisterInfo::AlwaysReservedRegs;
+
+BitVector SIRegisterInfo::getAlwaysReservedRegs() const {
+  // Already been calculated, so do not compute again.
+  if (AlwaysReservedRegs.size() == getNumRegs()) {
+    return AlwaysReservedRegs;
+  }
----------------
shiltian wrote:

```suggestion
  if (AlwaysReservedRegs.size() == getNumRegs())
    return AlwaysReservedRegs;
```

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