cps-ir: Merge variables based on the source variable's name.

cps-ir: Merge variables based on the source variable's name.

Previously we would merge based on the element, but that is a poor
strategy when multiple locals have the same name. E.g:

  for (int i = 0; i < X; ++i) {...}
  for (int i = 0; i < Y; ++i) {...}

The two `i` variables should obviously be merged, but they have
different elements.

BUG=
R=kmillikin@google.com

Committed: https://code.google.com/p/dart/source/detail?r=45134

[asgerf, 2015-04-14 08:47:01]

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

[Kevin Millikin (Google), 2015-04-14 08:57:53]

lgtm

[asgerf, 2015-04-14 10:51:50]

This caused a bug in dart2dart where a parameter would be substituted by a local
variable.

Changed treatment of parameters to avoid this.

[Kevin Millikin (Google), 2015-04-14 11:21:39]

LGTM.

[asgerf, 2015-04-14 12:14:10]

Committed patchset #3 (id:40001) manually as 45134 (presubmit successful).