MachineVerifier: Print name of failing subregister index (#129491)

I'm not sure of a good example to test the "does not fully support"
case.

Author: arsenm

llvm/lib/CodeGen/MachineVerifier.cpp

@@ -2749,13 +2749,15 @@ MachineVerifier::visitMachineOperand(const MachineOperand *MO, unsigned MONum) {
         if (!SRC) {
           report("Invalid subregister index for virtual register", MO, MONum);
           OS << "Register class " << TRI->getRegClassName(RC)
-             << " does not support subreg index " << SubIdx << '\n';
+             << " does not support subreg index "
+             << TRI->getSubRegIndexName(SubIdx) << '\n';
           return;
         }
         if (RC != SRC) {
           report("Invalid register class for subregister index", MO, MONum);
           OS << "Register class " << TRI->getRegClassName(RC)
-             << " does not fully support subreg index " << SubIdx << '\n';
+             << " does not fully support subreg index "
+             << TRI->getSubRegIndexName(SubIdx) << '\n';
           return;
         }
       }

llvm/test/MachineVerifier/AMDGPU/unsupported-subreg-index-aligned-vgpr-check.mir

@@ -19,7 +19,7 @@ body:             |
     ; CHECK-NEXT: - basic block: %bb.0
     ; CHECK-NEXT: - instruction: GLOBAL_STORE_DWORDX4_SADDR %0:vgpr_32, %1.sub16_sub17_sub18_sub19:vreg_512_align2, undef $sgpr8_sgpr9, 80, 0, implicit $exec :: (store (s128), addrspace 1)
     ; CHECK-NEXT: - operand 1:   %1.sub16_sub17_sub18_sub19:vreg_512_align2
-    ; CHECK-NEXT: Register class VReg_512_Align2 does not support subreg index 166
+    ; CHECK-NEXT: Register class VReg_512_Align2 does not support subreg index sub16_sub17_sub18_sub19
 
     ; CHECK: *** Bad machine code: Subtarget requires even aligned vector registers ***
     ; CHECK-NEXT: - function:    uses_invalid_subregister_for_regclass
@@ -33,7 +33,7 @@ body:             |
     ; CHECK-NEXT: - basic block: %bb.0
     ; CHECK-NEXT: - instruction: GLOBAL_STORE_DWORDX4_SADDR %0:vgpr_32, %2.sub16_sub17_sub18_sub19:vreg_512, undef $sgpr8_sgpr9, 80, 0, implicit $exec :: (store (s128), addrspace 1)
     ; CHECK-NEXT: - operand 1:   %2.sub16_sub17_sub18_sub19:vreg_512
-    ; CHECK-NEXT: Register class VReg_512 does not support subreg index 166
+    ; CHECK-NEXT: Register class VReg_512 does not support subreg index sub16_sub17_sub18_sub19
     S_NOP 0, implicit-def %2:vreg_512
     GLOBAL_STORE_DWORDX4_SADDR %0, %2.sub16_sub17_sub18_sub19, undef $sgpr8_sgpr9, 80, 0, implicit $exec :: (store (s128), addrspace 1)
     S_ENDPGM 0