Mark all loop conditions.

Mark all loop conditions.

Previously we marked compare operations that occurred as for loop
conditions so as to avoid inlining the floating-point code for them.
Begin marking all expressions that occur as any loop condition
(because they are never dead code).

Review URL: http://codereview.chromium.org/1074009

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@4227 ce2b1a6d-e550-0410-aec6-3dcde31c8c00