Document effect of constant folding on CASE.

Back-patch to all supported versions.

Laurenz Albe
Initialize pad bytes in GinFormTuple().

Every other core buffer page consumer initializes the bytes it furnishes
to PageAddItem().  For consistency, do the same here.  No back-patch;
regardless, we couldn't count on the fix so long as binary upgrade can
carry forward affected index builds.
Refactor aset.c and mcxt.c in preparation for Valgrind cooperation.

Move some repeated debugging code into functions and store intermediates
in variables where not presently necessary.  No code-generation changes
in a production build, and no functional changes.  This simplifies and
focuses the main patch.
Cooperate with the Valgrind instrumentation framework.

Valgrind "client requests" in aset.c and mcxt.c teach Valgrind and its
Memcheck tool about the PostgreSQL allocator.  This makes Valgrind
roughly as sensitive to memory errors involving palloc chunks as it is
to memory errors involving malloc chunks.  Further client requests in
PageAddItem() and printtup() verify that all bits being added to a
buffer page or furnished to an output function are predictably-defined.
Those tests catch failures of C-language functions to fully initialize
the bits of a Datum, which in turn stymie optimizations that rely on
_equalConst().  Define the USE_VALGRIND symbol in pg_config_manual.h to
enable these additions.  An included "suppression file" silences nominal
errors we don't plan to fix.

Reviewed in earlier versions by Peter Geoghegan and Korry Douglas.