Name type conversions from int to float and vice versa consistently,
and move them to the TurboAssembler, such that we can reuse them for
Liftoff.

R=jarin@chromium.org

Bug: v8:6600
Change-Id: Idced658a228eeb611dd4785aa277bd758c201eea
Reviewed-on: https://chromium-review.googlesource.com/1014037Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52620}