So far we only recognize the special

  NumberFloor(NumberDivide(lhs, rhs))

subgraph when both lhs and rhs are in the Unsigned32 range, and the
result is a PlainNumber. Extend this pattern matching to also cover

  NumberFloor(SpeculativeNumberDivide(lhs, rhs))

and to replace the NumberFloor with NumberToInt32 truncation if the
lhs value is in Signed32 range and the rhs is in Unsigned32 range.

R=jarin@chromium.org
BUG=v8:5267

Review-Url: https://codereview.chromium.org/2739573004
Cr-Commit-Position: refs/heads/master@{#43642}