This is a reland of f22156bf.

Original change's description:
> [Liftoff] Rename kConstant to kI32Const
>
> This makes it more clear that we only store i32 values as constants in
> the cache state.
>
> R=ahaas@chromium.org
>
> Bug: v8:6600
> Change-Id: I3022e963e864c9c533cbb673bd8502481389799b
> Reviewed-on: https://chromium-review.googlesource.com/853870
> Reviewed-by: Andreas Haas <ahaas@chromium.org>
> Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#50437}

Bug: v8:6600
Change-Id: I4c28f379eab525a0c86866c39b2e6b55f7c5ab1a
Reviewed-on: https://chromium-review.googlesource.com/857017
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50446}