X87: silence C++ compiler warnings.

To solence C++ compiler warnings because is_mutable_ is only for CHECK.

BUG=

Review URL: https://codereview.chromium.org/922673002

Cr-Commit-Position: refs/heads/master@{#26618}

src/x87/lithium-codegen-x87.h

@@ -408,6 +408,7 @@ class LCodeGen: public LCodeGenBase {
     int st(X87Register reg) { return st2idx(ArrayIndex(reg)); }
     void pop() {
       DCHECK(is_mutable_);
+      USE(is_mutable_);
       stack_depth_--;
     }
     void push(X87Register reg) {