[AMDGPU] Fix -Wunused-variable in SIInsertWaitcnts.cpp (NFC)

llvm-project/llvm/lib/Target/AMDGPU/SIInsertWaitcnts.cpp:1539:10:
 error: unused variable 'SWaitInst' [-Werror,-Wunused-variable]
    auto SWaitInst =
         ^
1 error generated.

Author: DamonFool

llvm/lib/Target/AMDGPU/SIInsertWaitcnts.cpp

@@ -1536,7 +1536,7 @@ bool WaitcntGeneratorGFX12Plus::createNewWaitcnt(
     if (Count == ~0u)
       continue;
 
-    auto SWaitInst =
+    [[maybe_unused]] auto SWaitInst =
         BuildMI(Block, It, DL, TII->get(instrsForExtendedCounterTypes[CT]))
             .addImm(Count);