This allows immediates to be encoded directly into instructions, rather than
mov-ing constants to registers first.

This patch only changes emit_{i64,i32}_add, other emit_ functions will be changed once
this approach has been approved.

Bug: v8:9038

Change-Id: I0c7306c2da0dae26f1c6e2465a9565adbf0bda84
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1524482
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60506}