Avoid descending into Expressions unnecessarily.

We know expressions can't contain @if or @switch statements inside of
them, so we can avoid recursing into them and run this check slightly
faster.

Change-Id: I60cd6dd40ddda74f7af259fd13e5ea0779982384
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/370076
Commit-Queue: John Stiles <johnstiles@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>

src/sksl/SkSLCompiler.cpp

@@ -1648,6 +1648,11 @@ void Compiler::verifyStaticTests(const Program& program) {
             return INHERITED::visitStatement(stmt);
         }
 
+        bool visitExpression(const Expression&) override {
+            // We aren't looking for anything inside an Expression, so skip them entirely.
+            return false;
+        }
+
     private:
         using INHERITED = ProgramVisitor;
         ErrorReporter* fReporter;