Reapply "A simpler scheme for garbage collection of ureachable phi inputs."

Reapply "A simpler scheme for garbage collection of ureachable phi inputs."

Fix two issues:

* Unreachable code elimination did not correctly identify all unreachable
  blocks.

* Loop detection in the register allocator relied on the block IDs.

R=vegorov@google.com
BUG=

Committed: https://code.google.com/p/dart/source/detail?r=12756

[Kevin Millikin (Google), 2012-09-20 13:48:17]

https://codereview.chromium.org/10956013/diff/1/runtime/vm/flow_graph_allocat...
File runtime/vm/flow_graph_allocator.cc (right):

https://codereview.chromium.org/10956013/diff/1/runtime/vm/flow_graph_allocat...
runtime/vm/flow_graph_allocator.cc:1131: // This algorithm relies on the
assumption that we emit blocks in reverse
Look here.

https://codereview.chromium.org/10956013/diff/1/runtime/vm/flow_graph_optimiz...
File runtime/vm/flow_graph_optimizer.cc (right):

https://codereview.chromium.org/10956013/diff/1/runtime/vm/flow_graph_optimiz...
runtime/vm/flow_graph_optimizer.cc:1932: // The successors may be reachable, but
only if this instruction is.  (We
And look here.

[Vyacheslav Egorov (Google), 2012-09-21 14:48:05]

lgtm

https://chromiumcodereview.appspot.com/10956013/diff/1/runtime/vm/flow_graph_...
File runtime/vm/flow_graph_optimizer.cc (right):

https://chromiumcodereview.appspot.com/10956013/diff/1/runtime/vm/flow_graph_...
runtime/vm/flow_graph_optimizer.cc:1935: BlockEntryInstr* entry = NULL;
->GetBlock() instead of custom loop