pgsql: Fixup some misusages of bms_num_members()

Fixup some misusages of bms_num_members()

It's a bit inefficient to test if a Bitmapset is empty by counting all the
members and seeing if that number is zero. It's much better just to use
bms_is_empty(). Likewise for checking if there are at least two members,
just use bms_membership(), which does not need to do anything more after
finding two members.

Discussion: https://postgr.es/m/CAApHDvpvwm_QjbDOb5xga%2BKmX9XkN9xQavNGm3SvDbVnCYOerQ%40mail.gmail.com
Reviewed-by: Tomas Vondra

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/c34605daed563fcade07a9f45bcf440459599c00

Modified Files
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src/backend/optimizer/path/clausesel.c | 3 +--
src/backend/statistics/dependencies.c | 8 ++++----
2 files changed, 5 insertions(+), 6 deletions(-)