[turbofan] Minor cleanups to lowering of typed array loads/stores.

TEST=unittests
R=dcarney@chromium.org

Review URL: https://codereview.chromium.org/680063004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24917 ce2b1a6d-e550-0410-aec6-3dcde31c8c00