regexec.c: utf8 doesn't match /i nonutf8 self

regexec.c: utf8 doesn't match /i nonutf8 self

This is a continuation of [perl #78464].  It fixes it also for the /i
flag.  After this, a character should match itself in the regrepeat
function, even if one is in utf8 and the other isn't, for both /i and
not.

The solution is to move the code for handling /i into the non-i
structure so that the decisions about utf8 are all in one place.  When
the string is in utf8, it uses the utf8-fold function.

This has the added effect of fixing a few cases where a utf8 string did
not match a fold in a non-utf8 pattern.  I haven't added tests for
these, as it only fixes a few cases where this is a problem, and I'm
working on a comprehensive solution to the problem, accompanied by
extensive tests.