Compute immediate dominators using SEMI-NCA.

Compute immediate dominators using SEMI-NCA.

This is a version of the Lengauer-Tarjan algorithm that is both simpler and
faster.

R=srdjan@google.com
BUG=
TEST=

Committed: https://code.google.com/p/dart/source/detail?r=5737

[Kevin Millikin (Google), 2012-03-19 23:24:59]

Dominators computation is unverified.  I'd like to have tests that verify it. 
We could verify it by having tests and using a different algorithm.  What do you
think?

We might want the dominator tree (not just pointers to immediate dominators). 
I've held off on that until we know for sure what we want.

Next steps will be: 1. Stop using this pass for unoptimized code, 2. implement
some sort of visualization of both flow graphs and dominator trees, and 3.
translate to SSA.

http://codereview.chromium.org/9729015/diff/1/runtime/vm/intermediate_language.h
File runtime/vm/intermediate_language.h (right):

http://codereview.chromium.org/9729015/diff/1/runtime/vm/intermediate_languag...
runtime/vm/intermediate_language.h:816: // a side effect of this function76, the
set of basic block predecessors
Oops.  The stray '76' is deleted.

[srdjan, 2012-03-21 20:57:50]

LGTM, make sure you run all frog code as test. 
Questions raised and plans proposed were discussed offline.

http://codereview.chromium.org/9729015/diff/1/runtime/vm/flow_graph_builder.cc
File runtime/vm/flow_graph_builder.cc (right):

http://codereview.chromium.org/9729015/diff/1/runtime/vm/flow_graph_builder.c...
runtime/vm/flow_graph_builder.cc:1707: intptr_t size = parent->length();
const intptr_t