pgsql: Refactor aset.c and mcxt.c in preparation for Valgrind cooperati

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.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/a855148a29b786b179308b3bd5c59fe5b67110d8

Modified Files
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src/backend/utils/mmgr/aset.c |   65 ++++++++++++++++++++++++++++-------------
src/backend/utils/mmgr/mcxt.c |   46 +++++++++++++++++------------
2 files changed, 73 insertions(+), 38 deletions(-)