[turbofan] Refactor the InstructionSelector tests.

Also fix some issues and improve test coverage.

TEST=compiler-unittests
BUG=v8:3489
LOG=y
R=jarin@chromium.org

Review URL: https://codereview.chromium.org/469743002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23111 ce2b1a6d-e550-0410-aec6-3dcde31c8c00