[Liftoff] Merge i32 comparisons with subsequent br_if

Most comparisons are followed by a br_if instruction. This CL detects
that for i32 comparisons and folds them with a following br_if,
avoiding to materialize the result of the comparison as 0 or 1 in a
register. This saves three out of the five instructions generated for
this pattern before.

Plus some drive-by cleanup.

R=jkummerow@chromium.org

Bug: v8:9831
Change-Id: I7bed47f2523b60b2bf301f074fe3132aea9a0636
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1862561
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64330}