ARM: Smi refactoring and improvements.

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: https://v8.googlecode.com/svn/branches/bleeding_edge@14724 ce2b1a6d-e550-0410-aec6-3dcde31c8c00