[turbofan] Add backend support for signed loads.

Also rename the arch opcodes to match their native counterparts.

TEST=compiler-unittests,cctest
R=jarin@chromium.org

Review URL: https://codereview.chromium.org/505713002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23345 ce2b1a6d-e550-0410-aec6-3dcde31c8c00