Remove most uses of HSubgraph as an argument.

Remove most uses of HSubgraph as an argument.

In a lot of cases where we were passing a subgraph (a pair of basic blocks),
we actually only needed a basic block.

Committed: http://code.google.com/p/v8/source/detail?r=6913

[Kevin Millikin (Chromium), 2011-02-23 10:41:13]

Simple next round of HSubgraph removal.

The next step after this is to remove all HSubgraph's member functions except
simple accessors, and to move the functionality to HBasicBlock, HGraph, or
HGraphBuilder as appropriate.

[fschneider, 2011-02-23 11:41:12]

LGTM.