Resolve

Resolve
http://www.nntp.perl.org/group/perl.perl5.porters/2007/06/msg125667.html
by reverting part of change #29354.

Unfortunately match vars after a /g match in scalar context will be
unsafe (again) after this, but such matches on long strings won't be
as diabolically slow.

Question: why does the new test in t/op/pat.t pass, but the same test
in t/op/reg_unsafe.t fail? (Latter is TODO for now)
p4raw-link: @29354 on //depot/perl: 58e23c8d7d24dd08c87b5d56819ad45527176c15

p4raw-id: //depot/perl@31451