[turbofan] Improve testability of the InstructionSelector.

[turbofan] Improve testability of the InstructionSelector.

Allow to pass the set of supported CPU features to
the InstructionSelector, so it can be tested without
messing with the command line flags.

Also add InstructionSelector sample for ia32.

TEST=cctest/test-instruction-selector,cctest/test-instruction-selector-{arm,ia32}
R=svenpanne@chromium.org

Committed: https://code.google.com/p/v8/source/detail?r=22876

[Benedikt Meurer, 2014-08-05 12:35:43]

PTAL

[Sven Panne, 2014-08-05 13:23:34]

lgtm

[Benedikt Meurer, 2014-08-05 13:27:05]

Committed patchset #2 manually as 22876 (presubmit successful).