Generate get_by_id for bracket access with constant string subscript.

Generate get_by_id for bracket access with constant string subscript.
<https://webkit.org/b/138663>

Reviewed by Michael Saboff.

Transform o["f"] into o.f when generating bytecode. This allows our JIT
to inline-cache those accesses instead of always dropping out to C++.

This is surprisingly common in real-web content, less so in benchmarks.
Interestingly, Speedometer does hit the optimization quite a bit.

* bytecompiler/NodesCodegen.cpp:
(JSC::BracketAccessorNode::emitBytecode):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@176035 268f45cc-cd09-0410-ab3c-d52691b4dbfc