Reduce register pressure in some runtime functions

Hoped to cure the spurious 3% regression we saw on beloved chromium-rel-win7-dual/sunspider after http://code.google.com/p/v8/source/detail?r=14236

R=mstarzinger@chromium.org
BUG=chromium:233441

Review URL: https://codereview.chromium.org/15297004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14722 ce2b1a6d-e550-0410-aec6-3dcde31c8c00