[Perl/perl5] d163af: gh18770: stop scanning for substrs after *COMMIT

[Hugo van der Sanden via perl5-changes]

  Branch: refs/heads/gh18770
  Home:   https://github.com/Perl/perl5
  Commit: d163af820d6a0c507a18e8ea4d3d2cc86ab6bebf
      
https://github.com/Perl/perl5/commit/d163af820d6a0c507a18e8ea4d3d2cc86ab6bebf
  Author: Hugo van der Sanden <h...@crypt.org>
  Date:   2021-05-07 (Fri, 07 May 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.