AMDGPU: Fix broken broken negative test for gfx950 assembler (llvm#129667) (llvm#129686)

Fix's not rejecting global_load_lds_dwordx3 and x4 on other targets.
The encoded versions of instructions should not touch
SubtargetPredicate,
and only AssemblerPredicate.
    
(cherry picked from commit f319a65)

Author: arsenm

llvm/lib/Target/AMDGPU/FLATInstructions.td

@@ -1873,7 +1873,7 @@ multiclass FLAT_Real_AllAddr_LDS<bits<7> op, bits<7> pre_gfx940_op,
     }
   }
 
-  let SubtargetPredicate = isGFX940Plus in {
+  let AssemblerPredicate = isGFX940Plus in {
     def _gfx940 : FLAT_Real_gfx940<op, !cast<FLAT_Pseudo>(NAME)>;
     def _SADDR_gfx940 : FLAT_Real_gfx940<op, !cast<FLAT_Pseudo>(NAME#"_SADDR")>;
   }

llvm/test/MC/AMDGPU/gfx950_asm_features.s

@@ -1,11 +1,10 @@
 // RUN: llvm-mc -triple=amdgcn -mcpu=gfx950 -show-encoding %s | FileCheck --check-prefix=GFX950 --strict-whitespace %s
-// xUN: not llvm-mc -triple=amdgcn -mcpu=gfx940 %s 2>&1 | FileCheck --check-prefixes=NOT-GFX950,GFX940 --implicit-check-not=error: %s
-// xUN: not llvm-mc -triple=amdgcn -mcpu=gfx90a %s 2>&1 | FileCheck --check-prefixes=NOT-GFX950,GFX90A --implicit-check-not=error: %s
-// xUN: not llvm-mc -triple=amdgcn -mcpu=gfx1010 %s 2>&1 | FileCheck --check-prefixes=NOT-GFX950,GFX10 --implicit-check-not=error: %s
+// RUN: not llvm-mc -triple=amdgcn -mcpu=gfx942 %s 2>&1 | FileCheck --check-prefixes=NOT-GFX950 --implicit-check-not=error: %s
+// RUN: not llvm-mc -triple=amdgcn -mcpu=gfx90a %s 2>&1 | FileCheck --check-prefixes=NOT-GFX950 --implicit-check-not=error: %s
+// RUN: not llvm-mc -triple=amdgcn -mcpu=gfx1010 %s 2>&1 | FileCheck --check-prefixes=NOT-GFX950 --implicit-check-not=error: %s
 
 // NOT-GFX950: :[[@LINE+2]]:{{[0-9]+}}: error: instruction not supported on this GPU
 // GFX950: global_load_lds_dwordx3 v[2:3], off     ; encoding: [0x00,0x80,0xf8,0xdd,0x02,0x00,0x7f,0x00]
-
 global_load_lds_dwordx3 v[2:3], off
 
 // NOT-GFX950: :[[@LINE+2]]:{{[0-9]+}}: error: