The proper fix would be to make TruncatingUseInfoFromRepresentation
respect tagged signed use representation, but requires extra work
to refine typing for all values that are stored into Smi fields.

Bug: chromium:791245
Change-Id: I83965bcc18a836d2c758a6a8b1477a4aa2c6133d
Reviewed-on: https://chromium-review.googlesource.com/808866Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Jaroslav Sevcik <jarin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49870}