This simplifies the handling of MachineTypes for parameters and returns
used in tests, and overall improves the regularity with which they
are handled in both tests and in CallDescriptor.

R=bmeurer@chromium.org, jarin@chromium.org
BUG=

Review URL: https://codereview.chromium.org/530783002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23638 ce2b1a6d-e550-0410-aec6-3dcde31c8c00