MachineVerifier: Print name of failing subregister index

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.mir

@@ -0,0 +1,15 @@
+# RUN: not --crash llc -mtriple=amdgcn -mcpu=gfx900 -run-pass=none -o /dev/null %s 2>&1 | FileCheck %s
+
+---
+name: unsupported_subreg_index
+body:             |
+  bb.0:
+    liveins: $vgpr0_vgpr1, $sgpr0_sgpr1, $agpr0_agpr1_agpr2_agpr3
+
+    ; CHECK: *** Bad machine code: Invalid subregister index for virtual register ***
+    undef %0.sub3:vreg_96 = COPY $vgpr0
+
+    ; CHECK: *** Bad machine code: Invalid subregister index for virtual register ***
+    $vgpr0 = COPY %0.sub3
+
+...