[Turbofan] Add USE for local used only by DCHECK.

LOG=N
BUG=v8:4124

Review-Url: https://codereview.chromium.org/2453813002
Cr-Commit-Position: refs/heads/master@{#40603}

src/compiler/register-allocator.cc

@@ -1936,6 +1936,7 @@ TopLevelLiveRange* LiveRangeBuilder::FixedFPLiveRangeFor(
   }
 
   DCHECK(index < num_regs);
+  USE(num_regs);
   TopLevelLiveRange* result = (*live_ranges)[index];
   if (result == nullptr) {
     result = data()->NewLiveRange(FixedFPLiveRangeID(index, rep), rep);