Perl_re_op_compile(): handle utf8 concating better

Perl_re_op_compile(): handle utf8 concating better

When concatting the list of arguments together to form a final pattern
string, the code formerly did a quick scan of all the args first, and
if any of them were SvUTF8, it set the (empty) destination string to UTF8
before concatting all the individual args. This avoided the pattern
getting upgraded to utf8 halfway through, and thus the indices for code
blocks becoming invalid.

However this was not 100% reliable because, as an "XXX" code comment of
mine pointed out, when overloading is involved it is possible for an arg
to appear initially not to be utf8, but to be utf8 when its value is
finally accessed. This results an obscure bug (as shown in the test added
for this commit), where literal /(?{code})/ still required 'use re
"eval"'.

The fix for this is to instead adjust the code block indices on the fly
if the pattern string happens to get upgraded to utf8. This is easy(er)
now that we have the new S_pat_upgrade_to_utf8() function.

As well as fixing the bug, this also simplifies the main concat loop in
the code, which will make it easier to handle interpolating arrays (e.g.
/@foo/) when we move the interpolation from the join op into the regex
engine itself shortly.