[Perl/perl5] 499aa1: gh18770: stop scanning for substrs after *COMMIT

[Hugo van der Sanden via perl5-changes]

  Branch: refs/heads/blead
  Home:   https://github.com/Perl/perl5
  Commit: 499aa13271ff03425a8258615a0702c5b830be8b
      
https://github.com/Perl/perl5/commit/499aa13271ff03425a8258615a0702c5b830be8b
  Author: Hugo van der Sanden <h...@crypt.org>
  Date:   2021-06-01 (Tue, 01 Jun 2021)

  Changed paths:
    M regcomp.c
    M t/re/opt.t

  Log Message:
  -----------
  gh18770: stop scanning for substrs after *COMMIT

*ACCEPT already avoids this (because it is "ENDLIKE"), but gets a
related fix to stop scanning for start class.