Improve performance of keyed loads/stores which have a HeapNumber index.

Improve performance of keyed loads/stores which have a HeapNumber index.

Some GWT compiled code results in array access that has a heap number (e.g. -0)
as an index. Until now this would result in a generic IC.

For example:

a[-0] === a[0] or

a[0.25 * 4] === a[1]

This change detects heap numbers that are representable as a smi
and converts them. As a result we can still use the fast keyed monomorphic
ICs. Optimized code already handles keyed access with a double-key efficiently.

As a result the frame rate on the reported benchmark improves by roughly 2x.

BUG=v8:1388,v8:1295
Review URL: https://chromiumcodereview.appspot.com/9837109

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@11282 ce2b1a6d-e550-0410-aec6-3dcde31c8c00