tidy up patten match copying code

tidy up patten match copying code

(no functional changes).

1. Remove some dead code from pp_split; it's protected by an assert
that it could never be called.

2. Simplify the flags settings for the call to CALLREGEXEC() in
pp_substcont: on subsequent matches we always set REXEC_NOT_FIRST,
which forces the regex engine not to copy anyway, so passing the
REXEC_COPY_STR is pointless, as is the conditional code to set it.

3. (whitespace change): split a conditional expression over 2 lines
for easier reading.