[AMDGPU] Do not attempt sgpr spills to vgpr, when it is disabled

[RamNalamothu]

Summary

This covers a path missed in https://reviews.llvm.org/D95768.

llvm/lib/Target/AMDGPU/SILowerSGPRSpills.cpp

for (MachineBasicBlock &MBB : MF) {
 
             // FIXME: change to enterBasicBlockEnd()
             RS->enterBasicBlock(MBB);
             TRI->eliminateFrameIndex(MI, 0, FIOp, RS.get());
             continue;
           }
         }
 
-        if (!TII->isSGPRSpill(MI))
+        if (!TII->isSGPRSpill(MI) || !TRI->spillSGPRToVGPR())
           continue;
 
         int FI = TII->getNamedOperand(MI, AMDGPU::OpName::addr)->getIndex();
         assert(MFI.getStackID(FI) == TargetStackID::SGPRSpill);
         if (FuncInfo->allocateSGPRSpillToVGPR(MF, FI)) {
           NewReservedRegs = true;
           bool Spilled = TRI->eliminateSGPRToVGPRSpillFrameIndex(MI, FI, nullptr);
           (void)Spilled;

llvm/test/CodeGen/AMDGPU/frame-setup-without-sgpr-to-vgpr-spills.ll

 ; RUN: llc -march=amdgcn -mcpu=gfx900 -verify-machineinstrs -amdgpu-spill-sgpr-to-vgpr=true < %s | FileCheck -check-prefixes=GCN,SPILL-TO-VGPR %s
 ; RUN: llc -march=amdgcn -mcpu=gfx900 -verify-machineinstrs -amdgpu-spill-sgpr-to-vgpr=false < %s | FileCheck -check-prefixes=GCN,NO-SPILL-TO-VGPR %s
 
 ; Check frame setup where SGPR spills to VGPRs are disabled or enabled.
 
 declare hidden void @external_void_func_void() #0
 
 ; GCN-LABEL: {{^}}callee_with_stack_and_call:
 ; SPILL-TO-VGPR:      buffer_store_dword v40, off, s[0:3], s32 offset:4 ; 4-byte Folded Spill
 ; SPILL-TO-VGPR:      v_writelane_b32 v40, s33, 2
+; SPILL-TO-VGPR:      v_writelane_b32 v40, s30, 0
+; SPILL-TO-VGPR:      v_writelane_b32 v40, s31, 1
 ; NO-SPILL-TO-VGPR:   v_mov_b32_e32 v0, s33
 ; NO-SPILL-TO-VGPR:   buffer_store_dword v0, off, s[0:3], s32 offset:12 ; 4-byte Folded Spill
+; NO-SPILL-TO-VGPR:   v_writelane_b32 v1, s30, 0
+; NO-SPILL-TO-VGPR:   v_writelane_b32 v1, s31, 1
+; NO-SPILL-TO-VGPR:   buffer_store_dword v1, off, s[0:3], s33 offset:4 ; 4-byte Folded Spill
 
 ; GCN:                s_swappc_b64 s[30:31], s[4:5]
 
 ; SPILL-TO-VGPR:      v_readlane_b32 s4, v40, 0
 ; SPILL-TO-VGPR:      v_readlane_b32 s5, v40, 1
+; NO-SPILL-TO-VGPR:   buffer_load_dword v1, off, s[0:3], s33 offset:4 ; 4-byte Folded Reload
 ; NO-SPILL-TO-VGPR:   v_readlane_b32 s4, v1, 0
 ; NO-SPILL-TO-VGPR:   v_readlane_b32 s5, v1, 1
 
 ; SPILL-TO-VGPR:      v_readlane_b32 s33, v40, 2
 ; NO-SPILL-TO-VGPR:   buffer_load_dword v0, off, s[0:3], s32 offset:12 ; 4-byte Folded Reload
 ; NO-SPILL-TO-VGPR:   v_readfirstlane_b32 s33, v0
 define void @callee_with_stack_and_call() #0 {
   %alloca = alloca i32, addrspace(5)
   store volatile i32 0, i32 addrspace(5)* %alloca
   call void @external_void_func_void()
   ret void
 }
 
 attributes #0 = { nounwind }