Previously we always lowered JSToBoolean(x:Number) to the subgraph
NumberLessThan(0.0, NumberAbs(x)), which deals with both 0, -0 and
NaNs appropriately. However this doesn't always generate the best,
especially when we can later derive from feedback that x is always
an Integral32 value, where the ideal code would be just a single
comparison to 0 w/o the absolute value computation.

R=mvstanton@chromium.org
BUG=v8:5267,v8:5270

Review-Url: https://codereview.chromium.org/2309953002
Cr-Commit-Position: refs/heads/master@{#39194}