tree-ir: Treat getInterceptor and identical as effectively constant.

tree-ir: Treat getInterceptor and identical as effectively constant.

Calls to getInterceptor would often block another expression from
propagating, which in turn can block other optimizations.

Before:

  while (true) {
    v0 = foo();
    if (P.identical(J.getInterceptor$n(i).$lt(i, v0), true)) { S1 }
    else { S2 [no continue] }

The call to J.getInterceptor prevents v0 = foo() from inlining.

After:

  while(P.identical(J.getInterceptor$n(i).$lt(i, foo()), true)) {
     S1
  }
  S2

BUG=
R=kmillikin@google.com

Committed: https://code.google.com/p/dart/source/detail?r=45161

[asgerf, 2015-04-14 13:45:48]

asgerf@google.com changed reviewers:
+ kmillikin@google.com

[asgerf, 2015-04-14 13:45:48]

https://codereview.chromium.org/1084783004/diff/1/pkg/compiler/lib/src/tree_i...
File pkg/compiler/lib/src/tree_ir/tree_ir_nodes.dart (right):

https://codereview.chromium.org/1084783004/diff/1/pkg/compiler/lib/src/tree_i...
pkg/compiler/lib/src/tree_ir/tree_ir_nodes.dart:182: bool isEffectivelyConstant
= false;
I am not overly enthusiastic about this design, but at least it's simple.

An alternative might be to ask the Backend if a given Element/Entity is known to
behave this way, but the Backends are already enormous. I don't want to feed the
monster.

Very open to ideas.

[Kevin Millikin (Google), 2015-04-15 09:57:52]

LGTM.  I imagine we will evolve it into something more general, but this is fine
for now.

[asgerf, 2015-04-15 11:24:41]

Committed patchset #2 (id:20001) manually as 45161 (presubmit successful).