pmruntime: make more use of Perl_re_op_compile

pmruntime: make more use of Perl_re_op_compile

In the compile-time, non-code branch of pmruntime(), rather than
assembling the pattern string ourselves from the list of constant strings,
just pass the whole thing to Perl_re_op_compile() (or whatever eng->op_comp
is in scope) to assemble.

This has two benefits. First it avoids duplicating the concatenation code
(which already has to be present in Perl_re_op_compile), and it provides
more information to that function; in particular so that we can identify
a late-bound compile-time pattern, i.e. $foo =~ 'string-not-regex'.