This introduces a helper class for the immediates to a "br_on_exn"
instruction. It is basically a wrapper around a pair of existing
immediates to simplify PC offset calculations. This is similar to what
is being done for e.g. {MemoryCopyImmediate} already.

R=clemensh@chromium.org
BUG=v8:8091

Change-Id: I85a552baa6bfa78304abc6cf9509aef365d47057
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1547865Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60586}