This change is motivated by SpiderMonkey's policy against bare
new/delete. (I also think it's just a nicer way to write this.)

R=jgruber@chromium.org

here is the same as the change I made in the equivalent SM code.

Note: I'm not importing regexp.cc into SpiderMonkey, but the change
Bug: v8:10303
Change-Id: I3c81727eb7dea9c0ec78241e3c82ffc9e7007827
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2091858
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66713}