Reinstate "regcomp.c: Remove unused data structure field"

Reinstate "regcomp.c: Remove unused data structure field"

This reverts commit c7995b20ccfbb4248b23aeae9fd1eab838852fcc, which
reverted 4b98096221966ea01c046f4f61b2dc4f60b534b9, thus reinstating
the latter commit.  It turns out that the error being chased down was
not due to this commit.

Its original message was:

This removes a field that is set in the inversion list data structure
and examined just once.  And that sole examiner is the function that
calls the function that does the set.  In other words X calls Y passing
it data D.  Y puts D into a structure.  Upon return from Y, X looks for
D in the structure.  No one else looks at D.  X might just as well have
looked at D directly, without involving Y, and without the structure
needing a space for D.