Cleanup of branch code generation (no change in functionality).

Cleanup of branch code generation (no change in functionality).

This CL is the first step in refactoring the way branches and comparisons
are generated.

1. Move helper functions from flow_graph_compiler_xyz.cc to
intermediate_language_xyz.cc.

2. Remove IL class ControlInstruction. It was only implemented by BranchInstr.
All functions provided are moved to BranchInstr.

3. When generating branch code for comparisons, pass the successor labels explicitly
instead of getting them from the branch. This will allow us to provide different labels
when materialize a bool value of a comparison.

4. Move some common code for IfThenElseInstr from the platform-specific files
into intermediate_language.cc and simplify it.

The goal is to enable if-conversion of arbitrary comparisons. Right now,
the code for == is hard-coded in IfThenElseInstr (and duplicated, too). This
means that e.g. "a < b ? 0 : 1" cannot be optimized.

As a result, IfThenElseInstr can be used to materialize the boolean  value of
a comparison. This way the complication of having ComparisonInstr both as a
normal instruction and as wrapped inside a BranchInstr can be simplified.
Comparisons would no longer appear as plain instructions in the IL, but only
wrapped inside either a Branch or an IfThenElse(true, false).

R=zra@google.com

Committed: https://code.google.com/p/dart/source/detail?r=30042

[zra, 2013-11-06 17:06:38]

lgtm

[fschneider, 2013-11-07 11:47:18]

Committed patchset #3 manually as r30042 (presubmit successful).