[turbofan] Introduce separate SelectLowering reducer.

[turbofan] Introduce separate SelectLowering reducer.

Split lowering of Select nodes into a separate graph reducer and be more
clever when lowering to diamonds, i.e. reuse diamonds that have the same
condition and only add more phis to it.

TEST=unittests
R=dcarney@chromium.org

Committed: https://code.google.com/p/v8/source/detail?r=25078

[Benedikt Meurer, 2014-11-03 14:57:08]

bmeurer@chromium.org changed reviewers:
+ dcarney@chromium.org

[Benedikt Meurer, 2014-11-03 14:57:08]

PTAL

[dcarney, 2014-11-03 15:10:18]

lgtm

[Benedikt Meurer, 2014-11-03 15:17:22]

Committed patchset #2 (id:20001) manually as 25078 (presubmit successful).