[AMDGPU][MC][GFX8+] Added syntactic sugar for 'vgpr index' operand of instructions s_set_gpr_idx_on and s_set_gpr_idx_mode

See bug 39331: https://bugs.llvm.org/show_bug.cgi?id=39331

Reviewers: artem.tamazov, arsenm

Differential Revision: https://reviews.llvm.org/D58288

llvm-svn: 354969
diff --git a/llvm/lib/Target/AMDGPU/Utils/AMDGPUAsmUtils.cpp b/llvm/lib/Target/AMDGPU/Utils/AMDGPUAsmUtils.cpp
index 67aa535..d8db2b0 100644
--- a/llvm/lib/Target/AMDGPU/Utils/AMDGPUAsmUtils.cpp
+++ b/llvm/lib/Target/AMDGPU/Utils/AMDGPUAsmUtils.cpp
@@ -85,5 +85,18 @@
 };
 
 } // namespace Swizzle
+
+namespace VGPRIndexMode {
+
+// This must be in sync with llvm::AMDGPU::VGPRIndexMode::Id enum members, see SIDefines.h.
+const char* const IdSymbolic[] = {
+  "SRC0",
+  "SRC1",
+  "SRC2",
+  "DST",
+};
+
+} // namespace VGPRIndexMode
+
 } // namespace AMDGPU
 } // namespace llvm