Simplify adding of computations to the flow graph.

Simplify adding of computations to the flow graph.

In preparation for collapsing BindInstr/DoInstr and flattening
Computations into the Instruction hierarchy.

Add a pair of functions EffectGraphVisitor::Bind and
EffectGraphVisitor::Do for adding computations to the flow graph that
have respectively one and zero uses.  This creates a single site where
we construct new BindInstr and a single site where we construct new
DoInstr (also, a single site where we construct new UseVal during flow
graph construction).

BUG=
TEST=

Committed: https://code.google.com/p/dart/source/detail?r=9407

[Florian Schneider, 2012-07-04 13:40:23]

This looks already much cleaner.

LGTM!