[llvm] [AMDGPU] Remove implicit definition of register group when restoring the last sub-register after a spill (PR #133986)

[Ryan Buchner via llvm-commits]

================
@@ -1928,7 +1928,9 @@ void SIRegisterInfo::buildSpillLoadStore(
       MIB->setAsmPrinterFlag(MachineInstr::ReloadReuse);
     }
 
-    if (NeedSuperRegImpOperand && (IsFirstSubReg || IsLastSubReg))
+    bool IsSrcDstDef = SrcDstRegState & RegState::Define;
+    if (NeedSuperRegImpOperand &&
+        (IsFirstSubReg || (IsLastSubReg && !IsSrcDstDef)))
----------------
bababuck wrote:

I believe the IsLastSubReg is needed for other flags, namely `RegState::Kill`. Removing it, the following change is seen in the test I wrote:
```
-    ; GFX942-NEXT: SCRATCH_STORE_DWORDX2_SADDR killed $vgpr12_vgpr13, $sgpr32, 48, 0, implicit $exec, implicit $flat_scr, implicit killed $vgpr0_vgpr1_vgpr2_vgpr3_vgpr4_vgpr5_vgpr6_vgpr7_vgpr8_vgpr9_vgpr10_vgpr11_vgpr12_vgpr13_vgpr14_vgpr15 :: (store (s64) into %stack.0 + 48, align 4, addrspace 5)
+    ; GFX942-NEXT: SCRATCH_STORE_DWORDX2_SADDR killed $vgpr12_vgpr13, $sgpr32, 48, 0, implicit $exec, implicit $flat_scr :: (store (s64) into %stack.0 + 48, align 4, addrspace 5)
```
This seems to be the analog of `implicit def` on the first sub-register.

https://github.com/llvm/llvm-project/pull/133986