libc: regex: rework unsafe pointer arithmetic

regcomp.c uses the "start + count < end" idiom to check that there are
"count" bytes available in an array of char "start" and "end" both point to.

This is fine, unless "start + count" goes beyond the last element of the
array. In this case, pedantic interpretation of the C standard makes the
comparison of such a pointer against "end" undefined, and optimizers from
hell will happily remove as much code as possible because of this.

An example of this occurs in regcomp.c's bothcases(), which defines
bracket[3], sets "next" to "bracket" and "end" to "bracket + 2". Then it
invokes p_bracket(), which starts with "if (p->next + 5 < p->end)"...

Because bothcases() and p_bracket() are static functions in regcomp.c, there
is a real risk of miscompilation if aggressive inlining happens.

The following diff rewrites the "start + count < end" constructs into "end -
start > count". Assuming "end" and "start" are always pointing in the array
(such as "bracket[3]" above), "end - start" is well-defined and can be
compared without trouble.

As a bonus, MORE2() implies MORE() therefore SEETWO() can be simplified a
bit.

PR: 252403
(cherry picked from commit d36b5dbe28d8ebab219fa29db533734d47f0c4a3)