[AMDGPU] Fix illegal VGPR to SGPR copy in FP elimination

[rampitec]

Summary

If a frame pointer was used in a S_MOV_B32 we were generating
V_MOV_B32 regardless.

[rampitec]

It needs more to make it working. We only have one offset value, so to use flat scratch we would need some pointer arithmetic on the FI and SP and then FI can occur in other SALU too.

llvm/lib/Target/AMDGPU/SIRegisterInfo.cpp

case AMDGPU::SI_SPILL_A1024_RESTORE: {
             FrameReg,
             TII->getNamedOperand(*MI, AMDGPU::OpName::offset)->getImm(),
             *MI->memoperands_begin(),
             RS);
       MI->eraseFromParent();
       break;
     }
 
+    case AMDGPU::S_MOV_B32:
+      if (FrameReg) {
+        MI->getOperand(FIOperandNum).ChangeToRegister(FrameReg, false);
+        break;
+      }
+      LLVM_FALLTHROUGH;
+
     default: {
       const DebugLoc &DL = MI->getDebugLoc();
       bool IsMUBUF = TII->isMUBUF(*MI);
 
       if (!IsMUBUF && !MFI->isEntryFunction()) {
         // Convert to a swizzled stack address by scaling by the wave size.
         //
         // In an entry function/kernel the offset is already swizzled.

llvm/test/CodeGen/AMDGPU/scratch-store-sgpr-fp.mir

+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py
+# RUN: llc -march=amdgcn -mcpu=gfx900 -verify-machineinstrs -run-pass=prologepilog %s -o - | FileCheck -check-prefix=GCN %s
+
+---
+name:            s_mov_from_fi
+tracksRegLiveness: true
+stack:
+  - { id: 0, type: default, offset: 0, size: 40, alignment: 4,
+      stack-id: default }
+machineFunctionInfo:
+  stackPtrOffsetReg: '$sgpr32'
+body:             |
+  bb.0.entry:
+    liveins: $sgpr30_sgpr31
+
+    ; GCN-LABEL: name: s_mov_from_fi
+    ; GCN: liveins: $sgpr30_sgpr31
+    ; GCN: renamable $sgpr4 = S_MOV_B32 $sgpr32
+    ; GCN: SCRATCH_STORE_DWORD_SADDR killed undef $vgpr1, killed renamable $sgpr4, 0, 0, 0, 0, implicit $exec, implicit $flat_scr
+    ; GCN: S_SETPC_B64_return killed renamable $sgpr30_sgpr31
+    renamable $sgpr4 = S_MOV_B32 %stack.0
+    SCRATCH_STORE_DWORD_SADDR killed undef $vgpr1, killed renamable $sgpr4, 0, 0, 0, 0, implicit $exec, implicit $flat_scr
+    S_SETPC_B64_return killed renamable $sgpr30_sgpr31
+
+...