[llvm] [AMDGPU] Create AMDGPUMnemonicAlias tablegen class (PR #89288)

[Jay Foad via llvm-commits]

================
@@ -2456,13 +2456,19 @@ class get_BUF_ps<string name> {
 
 // gfx11 instruction that accept both old and new assembler name.
 class Mnem_gfx11_gfx12 <string mnemonic, string real_name> :
-  MnemonicAlias<mnemonic, real_name>, Requires<[isGFX11Plus]>;
+    AMDGPUMnemonicAlias<mnemonic, real_name> {
----------------
jayfoad wrote:

It's a shame that so many cases like this get longer not shorter.

https://github.com/llvm/llvm-project/pull/89288