[vectorizer] Add some flags which are useful for conducting experiments

Description

[vectorizer] Add some flags which are useful for conducting experiments
with the unrolling behavior in the loop vectorizer. No functionality
changed at this point.

These are a bit hack-y, but talking with Hal, there doesn't seem to be
a cleaner way to easily experiment with different thresholds here and he
was also interested in them so I wanted to commit them. Suggestions for
improvement are very welcome here.

Details

Committed
chandlercJan 27 2014, 3:12 AM
Parents
rL200211: [vectorizer] Fix a trivial oversight where we always requested the
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