regcomp.c: Slightly relax restriction of SIMPLE nodes

regcomp.c: Slightly relax restriction of SIMPLE nodes

Currently all EXACTish nodes that are SIMPLE must be a single UTF-8
invariant character.  It turns out that the code works not just for
these, but for all Latin1 characters (when the pattern isn't UTF-8)
except the SHARP S under /d folding.

SIMPLE nodes allow for better optimization possibilities, such as CURLY
instead of CURLYM.

There is still a discrepancy in that non-EXACTish nodes that match a
single character, such as the dot (SANY), can be SIMPLE, but EXACTish
nodes have to be just a single byte.