Refactoring:
* consistent use of SmiTag/Untag
* added a few Smi macros and helpers
Improvements
* small optimisations (e.g. merging untag and cmp #0)
* added fixed point to double conversion instructions for simpler conversions
More on the last point: a Smi can be seen as a fixed point number with the
a one bit fractional part. Fixed to double instructions allow us to convert
a Smi to a double without untagging.
BUG=none
TEST=none
Review URL: https://chromiumcodereview.appspot.com/15085026
git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@14724 ce2b1a6d-e550-0410-aec6-3dcde31c8c00