PPC: Fix compilation error on mtcrf Sim

Change-Id: Ic4dff5209279f51ed75c76f2a0b159c727fb6164
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2767198Reviewed-by: Junliang Yan <junyan@redhat.com>
Reviewed-by: Joran Siu <joransiu@ca.ibm.com>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73483}

src/execution/ppc/simulator-ppc.cc

@@ -3885,6 +3885,7 @@ void Simulator::ExecuteGeneric(Instruction* instr) {
       int rs = instr->RSValue();
       uint8_t fxm = instr->Bits(19, 12);
       DCHECK_EQ(fxm, 0xFF);
+      USE(fxm);
       condition_reg_ = static_cast<int32_t>(get_register(rs));
       break;
     }