There was already a lookahead implementation in Liftoff for the case
where a comparison was followed by kExprBrIf. This CL extends this
lookahead implementation to kExprIf as well. This extension reduces the
size of the code generated by Liftoff in the Epic benchmark by 1.5%.

R=clemensb@chromium.org

Bug: v8:11873, v8:11862
Change-Id: If4428bdd64eedcdd6dc543efc3b9945cbd8be3cc
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2953322Reviewed-by: Clemens Backes <clemensb@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75088}