This CL removes the last bit of Float64 math that was left, by
adding a TypeGuard node to help the typer figure out that stuff
actually fits in Int32 operations.

This should improve the inline performance a little bit more.

Change-Id: I35bf3445e6c73c9433d5ff5e177d6bb3c8b82071
Reviewed-on: https://chromium-review.googlesource.com/1151635Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Théotime Grohens <theotime@google.com>
Cr-Commit-Position: refs/heads/master@{#54743}