ac/llvm: fix invalid use of unreachable in ac_build_atomic_rmw()

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6936>

src/amd/llvm/ac_llvm_helper.cpp

@@ -310,7 +310,7 @@ LLVMValueRef ac_build_atomic_rmw(struct ac_llvm_context *ctx, LLVMAtomicRMWBinOp
       binop = llvm::AtomicRMWInst::UMin;
       break;
    default:
-      unreachable(!"invalid LLVMAtomicRMWBinOp");
+      unreachable("invalid LLVMAtomicRMWBinOp");
       break;
    }
    unsigned SSID = llvm::unwrap(ctx->context)->getOrInsertSyncScopeID(sync_scope);