Subzero: Fix g++ warnings.

This tries to use the same -W options that the buildbots use, to reduce the amount of warning spam in the logs.

BUG= none
R=jpp@chromium.org

Review URL: https://codereview.chromium.org/1575873006 .
diff --git a/src/IceCfgNode.cpp b/src/IceCfgNode.cpp
index f23ae0f..ba50afb 100644
--- a/src/IceCfgNode.cpp
+++ b/src/IceCfgNode.cpp
@@ -526,6 +526,7 @@
           }
         }
         assert(Found);
+        (void)Found;
       }
       // Now that a cycle (if any) has been broken, create the actual
       // assignment.