Experimental: more code generator changes....

Experimental: more code generator changes.

1. Enable virtual frame/register allocator for for loops.

2. Spill all registers before the stack check code. We were always
   spilled before simply due to it being the function entry.  Now that
   we handle for loops, that is not necessarily the case.

3. Label blocks first reached via JumpTarget::Bind with a 'mergable'
   frame in case they are the later target of a backward CFG edge.  We
   didn't have non-spilled code at backward CFG edges before.

4. Avoid generating a branch to a jump for JumpTarget::Branch in the
   case that there is no code necessary to make the expected frame
   mergable.

5. Remove some overzealous spilling.

Committed: http://code.google.com/p/v8/source/detail?r=951

[Kevin Millikin (Chromium), 2008-12-08 14:40:48]

This change includes my previous one.  Take a look at this one instead if you
haven't already started, otherwise I'll take comments on the other one too (ie,
whichever is most convenient for you).

[Kevin Millikin (Chromium), 2008-12-09 09:51:45]

I've uploaded a new patch set that fixes a bug in the previous one.  After
jumping to a label, the frame becomes NULL.  In that case, the register
allocator needs to be informed that it is free to allocate the register
references previously held by that frame.

[William Hesse, 2008-12-09 12:35:42]

LGTM