Clobber non-live temporaries on some paths involving a possible slow-path allocation.

R=vegorov@google.com
BUG=

Review URL: https://codereview.chromium.org//783103003

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@42239 260f80e4-7a28-3924-810f-c04153c831b5
Revert "Disable the irregexp backend."

This reverts commit http://code.google.com/p/dart/source/detail?r=41999

R=fschneider@google.com
BUG=

Review URL: https://codereview.chromium.org//774933009

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@42240 260f80e4-7a28-3924-810f-c04153c831b5