3 small fixes:
  - Fix erroneous use of reg t6 in macro-assembler.
  - Minor optimization to overflow-check macros.
  - Fix un-init var use (typo) in simulator FPCall handling.

None of these affected test cases.

BUG=
TEST=

Review URL: http://codereview.chromium.org//7236025
Patch from Paul Lind <plind44@gmail.com>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@8418 ce2b1a6d-e550-0410-aec6-3dcde31c8c00