The NumberFloor operator matches exactly the semantics of the Math.floor
builtin on Numbers. It uses hardware rounding instructions if available,
but provides a full fallback solution that is compatible with Math.floor.
The lowering is optimizable based on types if needed later, i.e. we
already optimize it for the case that the input is already an Integer
(in the EcmaScript sense, including NaN and -0), but we could add more
optimizations, like combining NumberFloor and NumberDivide in the
future, if necessary.

R=jarin@chromium.org
BUG=v8:2890,v8:4059
LOG=n

Review URL: https://codereview.chromium.org/1843533003

Cr-Commit-Position: refs/heads/master@{#35090}