For debugging code, disable opcode merging. Otherwise, the effect of the
first merged opcode would not be observable when stepping.

R=thibaudm@chromium.org

Bug: v8:10350
Change-Id: Id656c9dee8f9676bf3d7881f3782e5ead76b5e71
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2306802
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68960}