Code generation for multiply-by-constant-smi....

Code gen improvements for multiply and logical shift.

Code generation for multiply-by-constant-smi:

For positive constant values the negative-zero check is eliminated.
For negative constant values the negative-zero check is simpified.

Special cases for constants -1, 0, and 1.

Improve UnarySubStub for Smi case to use the jo/jz trick.

Improved code sequence for  x >>> k.  For k = 0/1/n the old inline
code was 8/8/6 instructions, is now 2/4/5 instructions.

[Mads Ager (chromium), 2009-12-07 08:19:56]

Kevin, could you have a look at this one?