pgsql: Deduplicate code between slot_getallattrs() and slot_getsomeattr

Deduplicate code between slot_getallattrs() and slot_getsomeattrs().

Code in slot_getallattrs() is the same as if slot_getsomeattrs() is
called with number of attributes specified in the tuple
descriptor. Implement it that way instead of duplicating the code
between those two functions.

This is part of a patchseries abstracting TupleTableSlots so they can
store arbitrary forms of tuples, but is a nice enough cleanup on its
own.

Author: Ashutosh Bapat
Reviewed-By: Andres Freund
Discussion: https://postgr.es/m/20180220224318.gw4oe5jadhpmcdnm@alap3.anarazel.de

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/88ebd62fcc2ea7c55c0858f6dd4800d51383529f

Modified Files
--------------
src/backend/access/common/heaptuple.c | 45 -----------------------------------
src/include/executor/tuptable.h | 13 +++++++++-
2 files changed, 12 insertions(+), 46 deletions(-)