AMDGPU: Handle V->A MFMA copy from case with immediate src2

Handle a special case for copies from AGPR VGPR on the MFMA inputs.
If the "input" is really a subregister def, we will not see the
usual copy to VGPR for src2, only the read of the subregister def.
Not sure if this pattern appears in practice.

Author: arsenm

llvm/lib/Target/AMDGPU/AMDGPURewriteAGPRCopyMFMA.cpp

@@ -370,8 +370,11 @@ bool AMDGPURewriteAGPRCopyMFMAImpl::tryFoldCopiesFromAGPR(
     Register CopyDstReg = UseMI.getOperand(0).getReg();
     if (!CopyDstReg.isVirtual())
       continue;
+    for (MachineOperand &CopyUseMO : MRI.reg_nodbg_operands(CopyDstReg)) {
+      if (!CopyUseMO.readsReg())
+        continue;
 
-    for (MachineInstr &CopyUseMI : MRI.use_instructions(CopyDstReg)) {
+      MachineInstr &CopyUseMI = *CopyUseMO.getParent();
       if (isRewriteCandidate(CopyUseMI)) {
         if (tryReassigningMFMAChain(CopyUseMI, CopyDstReg,
                                     VRM.getPhys(CopyDstReg)))

llvm/test/CodeGen/AMDGPU/rewrite-vgpr-mfma-to-agpr-copy-from.mir

@@ -187,8 +187,8 @@ body:             |
     ; CHECK-NEXT: [[COPY1:%[0-9]+]]:av_64_align2 = COPY $vgpr0_vgpr1
     ; CHECK-NEXT: [[COPY2:%[0-9]+]]:av_64_align2 = COPY $vgpr2_vgpr3
     ; CHECK-NEXT: [[GLOBAL_LOAD_DWORDX4_:%[0-9]+]]:areg_128_align2 = GLOBAL_LOAD_DWORDX4 [[COPY]], 0, 0, implicit $exec :: (load (s128), addrspace 1)
-    ; CHECK-NEXT: [[COPY3:%[0-9]+]]:vreg_128_align2 = COPY [[GLOBAL_LOAD_DWORDX4_]]
-    ; CHECK-NEXT: [[COPY3:%[0-9]+]].sub0_sub1:vreg_128_align2 = V_MFMA_F64_4X4X4F64_vgprcd_e64 [[COPY1]], [[COPY2]], 0, 0, 0, 0, implicit $mode, implicit $exec
+    ; CHECK-NEXT: [[COPY3:%[0-9]+]]:areg_128_align2 = COPY [[GLOBAL_LOAD_DWORDX4_]]
+    ; CHECK-NEXT: [[COPY3:%[0-9]+]].sub0_sub1:areg_128_align2 = V_MFMA_F64_4X4X4F64_e64 [[COPY1]], [[COPY2]], 0, 0, 0, 0, implicit $mode, implicit $exec
     ; CHECK-NEXT: GLOBAL_STORE_DWORDX4 [[COPY]], [[COPY3]], 0, 0, implicit $exec :: (store (s128), addrspace 1)
     ; CHECK-NEXT: SI_RETURN
     %0:vreg_64_align2 = COPY $vgpr4_vgpr5