[llvm] [AMDGPU] Restore SP from saved-FP or saved-BP (PR #124007)

[Matt Arsenault via llvm-commits]

================
@@ -1256,6 +1256,17 @@ void SIFrameLowering::emitEpilogue(MachineFunction &MF,
   Register FramePtrReg = FuncInfo->getFrameOffsetReg();
   bool FPSaved = FuncInfo->hasPrologEpilogSGPRSpillEntry(FramePtrReg);
 
+  if (RoundedSize != 0) {
+    if (TRI.hasBasePointer(MF))
+      BuildMI(MBB, MBBI, DL, TII->get(AMDGPU::COPY), StackPtrReg)
+          .addReg(TRI.getBaseRegister())
+          .setMIFlag(MachineInstr::FrameDestroy);
+    else if (hasFP(MF))
+      BuildMI(MBB, MBBI, DL, TII->get(AMDGPU::COPY), StackPtrReg)
+          .addReg(FramePtrReg)
+          .setMIFlag(MachineInstr::FrameDestroy);
+  }
----------------
arsenm wrote:

```suggestion
    if (TRI.hasBasePointer(MF)) {
      BuildMI(MBB, MBBI, DL, TII->get(AMDGPU::COPY), StackPtrReg)
          .addReg(TRI.getBaseRegister())
          .setMIFlag(MachineInstr::FrameDestroy);
    } else if (hasFP(MF)) {
      BuildMI(MBB, MBBI, DL, TII->get(AMDGPU::COPY), StackPtrReg)
          .addReg(FramePtrReg)
          .setMIFlag(MachineInstr::FrameDestroy);
    }  
  }
```

https://github.com/llvm/llvm-project/pull/124007