Fix bug in x64 RegExp detecting start of string.

Also add missing MIPS case in regexp tracer.

Fixes issues v8:1748 and v8:1746

BUG=v8:1748, v8:1746
TEST=mjsunit/regress/regress-1748.js

Review URL: http://codereview.chromium.org/8116001

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@9504 ce2b1a6d-e550-0410-aec6-3dcde31c8c00