Make constants computations instead of values.

Make constants computations instead of values.

This change requires a pool of global constants that don't have an easy
place to go in the instruction stream.  Currently, that is only the constant
null used to mark eliminated phis in deoptimization environments.

R=fschneider@google.com
BUG=

Committed: https://code.google.com/p/dart/source/detail?r=11518

[Kevin Millikin (Google), 2012-08-29 09:56:00]

https://chromiumcodereview.appspot.com/10894034/diff/1/runtime/vm/flow_graph.cc
File runtime/vm/flow_graph.cc (right):

https://chromiumcodereview.appspot.com/10894034/diff/1/runtime/vm/flow_graph....
runtime/vm/flow_graph.cc:507: start_env.Add(graph_entry_->constant_null());
This is a bit dodgy, because there are now multiple occurrences of this single
definition.  I can fix that by making a proper environment instead of a list of
definitions here.

[Florian Schneider, 2012-08-29 11:07:48]

lgtm

https://chromiumcodereview.appspot.com/10894034/diff/1/runtime/vm/intermediat...
File runtime/vm/intermediate_language.cc (right):

https://chromiumcodereview.appspot.com/10894034/diff/1/runtime/vm/intermediat...
runtime/vm/intermediate_language.cc:110: constant_null_(new
BindInstr(BindInstr::kUsed,
Maybe break after the first '(' here helps?

https://chromiumcodereview.appspot.com/10894034/diff/1/runtime/vm/intermediat...
runtime/vm/intermediate_language.cc:424: return;
No need for a return here.

[Kevin Millikin (Google), 2012-08-29 12:43:44]

https://chromiumcodereview.appspot.com/10894034/diff/1/runtime/vm/intermediat...
File runtime/vm/intermediate_language.cc (right):

https://chromiumcodereview.appspot.com/10894034/diff/1/runtime/vm/intermediat...
runtime/vm/intermediate_language.cc:424: return;
On 2012/08/29 11:07:48, Florian Schneider wrote:
> No need for a return here.

Actually, this whole function seems kind of pointless now.  I'll just get rid of
it.