[AMDGPU] Remove useless aliases for FLAT instructions. NFC. (llvm#87462)

We were generating "" (the empty string) as an alias for a bunch of FLAT
instructions, which had no effect except to cause tablegen to generate
some very long if-else chains in the generate AsmMatcher.

Author: jayfoad

llvm/lib/Target/AMDGPU/FLATInstructions.td

@@ -2558,7 +2558,7 @@ multiclass VFLAT_Real_Base_gfx12<bits<8> op,
 
 multiclass VFLAT_Real_Atomics_gfx12<bits<8> op,
                                     string name = get_FLAT_ps<NAME>.Mnemonic,
-                                    string alias = ""> :
+                                    string alias = name> :
   VFLAT_Real_Base_gfx12<op, name, alias> {
   defm _RTN : VFLAT_Real_gfx12<op, name>;
 }
@@ -2581,7 +2581,7 @@ multiclass VGLOBAL_Real_AllAddr_gfx12_w64<bits<8> op,
 
 multiclass VGLOBAL_Real_Atomics_gfx12<bits<8> op,
                                       string name = get_FLAT_ps<NAME>.Mnemonic,
-                                      string alias = ""> :
+                                      string alias = name> :
   VGLOBAL_Real_AllAddr_gfx12<op, name, alias> {
   defm _RTN : VFLAT_Real_gfx12<op, name>;
   defm _SADDR_RTN : VFLAT_Real_gfx12<op, name>;