Introduce bms_offset_members() function

Introduce bms_offset_members() function

Effectively, a function to bitshift members by the specified number of
bits.  We have various fragments of code doing this manually with a
bms_next_member() -> bms_add_member() loop.  We can do this more
efficiently in terms of CPU and memory allocation by making a new
Bitmapset and bitshifting in the words of the old set to populate it.

Author: David Rowley <dgrowleyml@gmail.com>
Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Reviewed-by: Greg Burd <greg@burd.me>
Discussion: https://postgr.es/m/CAApHDvq=eEdw2Qp+aSzSOtTSe+h0fnVQ55CcTNqBkLDYiRZmxw@mail.gmail.com