C 1.45_14: support utf8 qr

B::PMOP->precomp does not set the utf8 flag, so
we check for utf8 chars in compiled qr strings by ourselves
and set the UTF8 flags then.

Fixes issue 333 and issue 338, and was checked with qr tests
20 39 44 71 131 143 1431 1432 330 333 338