This unblocks the checks in the SimplifiedLowering that whenever we
store something as TaggedSigned, the input type should at least be
Type::SignedSmall.

Bug: chromium:791245
Change-Id: Ice6e55c2c6584c0ff60c1e033ba755c8863af32a
Reviewed-on: https://chromium-review.googlesource.com/808104
Commit-Queue: Jaroslav Sevcik <jarin@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49856}