Don't rewrite unreachable statements

I'd like to change the parser to not create those AST nodes in
the first place. To get there, I'm skipping visiting of those nodes
in the existing visitors.

With this change, there is only one visitor in asm-to-wasm left that
actually visits those nodes, and seemingly depends on it.

R=adamk@chromium.org
BUG=v8:6312

Change-Id: I0837fdd97cf4c1baefa2d7fd76eddd90ad00b1df
Reviewed-on: https://chromium-review.googlesource.com/493167Reviewed-by: Adam Klein <adamk@chromium.org>
Commit-Queue: Jochen Eisinger <jochen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45044}

src/ast/ast-expression-rewriter.cc

@@ -31,7 +31,7 @@ void AstExpressionRewriter::VisitDeclarations(Declaration::List* declarations) {
 void AstExpressionRewriter::VisitStatements(ZoneList<Statement*>* statements) {
   for (int i = 0; i < statements->length(); i++) {
     AST_REWRITE_LIST_ELEMENT(Statement, statements, i);
-    // Not stopping when a jump statement is found.
+    if (statements->at(i)->IsJump()) break;
   }
 }