Begin changing Hydrogen branch instructions.

Begin changing Hydrogen branch instructions.

Rename HBranch (the instruction that coerces an arbitrary HValue to
control flow) to HTest to free up the term Branch to refer to any
control instruction with two successors.

Change the virtual FirstSuccessor and SecondSuccessor functions on
control instructions to a pair of data members.

Committed: http://code.google.com/p/v8/source/detail?r=6417

[Kevin Millikin (Chromium), 2011-01-18 12:48:48]

The immediate goal is to get rid of HValue::EmitAtUses and the way we use it to
special case for certain HInstruction's occurring as the operand of a branch. 
Ultimately, those instructions will *be* branches themselves.  This is a first
step toward that.

[fschneider, 2011-01-18 14:02:43]

LGTM.

Remember to file an issue porting your changes to the other platforms.