pgsql: Add bms_prev_member function

Add bms_prev_member function

This works very much like the existing bms_last_member function, only it
traverses through the Bitmapset in the opposite direction from the most
significant bit down to the least significant bit.  A special prevbit value of
-1 may be used to have the function determine the most significant bit.  This
is useful for starting a loop.  When there are no members less than prevbit,
the function returns -2 to indicate there are no more members.

Author: David Rowley
Discussion: https://postgr.es/m/CAKJS1f-K=3d5MDASNYFJpUpc20xcBnAwNC1-AOeunhn0OtkWbQ@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/5c0675215e153ba1297fd494b34af2fdebd645d1

Modified Files
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src/backend/nodes/bitmapset.c | 95 +++++++++++++++++++++++++++++++++++++++++++
src/include/nodes/bitmapset.h |  1 +
2 files changed, 96 insertions(+)