[regex] Small cleanup in subexpression handling

[Paolo Bonzini]

This cleanup has two parts.  The first is the remove re_dfa_t's subexps 
field, which is only used to test for backreferences used within their 
definition like in (x\1).  It can be replaced by a simple bitset.

The second is to make OP_BACK_REF 0-based (i.e. \1 corresponds to 
opr.idx==0) like OP_OPEN_SUBEXP and OP_CLOSE_SUBEXP are.  This 
eliminates a good deal of compensation in regexec.c, with no runtime 
effects but simplifying some expression.

Since I was at it, I removed a duplicate conditional in 
check_subexp_limits.  Outer ifs guarantee that (type == OP_OPEN_SUBEXP 
|| type == OP_CLOSE_SUBEXP) && ent->subexp_to != str_idx.

Tested with valgrind to avoid stupid off-by-one mistakes like the one 
Jakub found yesterday.

Paolo
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