regexec.c: regrepeat() change default: handling

regexec.c: regrepeat() change default: handling

The core of this function is a large switch() statement.  Prior to this
commit, the default: case of it assumed that this meant that the
node-type was a zero-length node.  It is safer to explicitly mention
all the cases you expect to handle, and have the default fail so that
you know sooner that you forgot to handle a situation.  That is how all
other switch statements in this file are set up, and now this one is
too.

I added cases for all the node types in regcomp.c that are SIMPLE, and
hence could conceivably be handled by regrepeat(), and changed the
default: case to croak.

I considered doing an #ifdef DEBUGGING around this, but no other
switches() in the file do so, so I followed that paradigm.