[AMDGPU] Fix AGPR_32 reg assign for mfma scale ops

In MFMA rewrite pass, prevent AGPR_32 reg class assignment for scale
operands, not permitted by instruction format.

Author: hjagasiaAMD

llvm/lib/Target/AMDGPU/AMDGPURewriteAGPRCopyMFMA.cpp

@@ -302,6 +302,10 @@ bool AMDGPURewriteAGPRCopyMFMAImpl::attemptReassignmentsToAGPR(
     const TargetRegisterClass *EquivalentAGPRRegClass =
         TRI.getEquivalentAGPRClass(MRI.getRegClass(InterferingReg));
 
+    // Do not reassign scale operands
+    if (EquivalentAGPRRegClass == &AMDGPU::AGPR_32RegClass)
+      return false;
+
     MCPhysReg Assignable = AMDGPU::NoRegister;
     if (EquivalentAGPRRegClass->contains(PrefPhysReg) &&
         LRM.checkInterference(ReassignLI, PrefPhysReg) ==

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

@@ -1,6 +1,6 @@
-# RUN: not llc -mtriple=amdgcn-amd-amdhsa -mcpu=gfx950 -run-pass=greedy,amdgpu-rewrite-agpr-copy-mfma -verify-machineinstrs -o - %s 2>&1 | FileCheck %s
-# CHECK: Illegal virtual register for instruction
-# CHECK: Expected a VGPR_32 register, but got a AGPR_32 register
+# RUN: llc -mtriple=amdgcn-amd-amdhsa -mcpu=gfx950 -run-pass=greedy,amdgpu-rewrite-agpr-copy-mfma -verify-machineinstrs -o - %s 2>&1 | FileCheck %s
+# CHECK-NOT: Illegal virtual register for instruction
+# CHECK-NOT: Expected a VGPR_32 register, but got a AGPR_32 register
 
 # Test for issue in amdgpu-rewrite-agpr-copy-mfma, which reassigns scale operand
 # in vgpr_32 register to agpr_32, not permitted by instruction format.