The LoopVariableOptimizer didn't understand NumberAdd, NumberSubtract
and friends, and so Phis in Loops that are generated in the call
reducer (like for Array.prototype.shift) got bad types, leading to
unnecessary double computations.

Bug: v8:8015
Change-Id: I9686f86682c106ef84f2760fed9948ce54d7cd9a
Reviewed-on: https://chromium-review.googlesource.com/1224371Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55859}