[MASSMAIL]pgsql: ALTER TABLE: rework determination of access method ID

ALTER TABLE: rework determination of access method ID

Avoid setting an access method OID for relation kinds that don't take
one.  Code review for new feature added in 374c7a229042.

Author: Justin Pryzby <pryzby@telsasoft.com>
Reported-by: Alexander Lakhin <exclusion@gmail.com>
Discussion: https://postgr.es/m/e5516ac1-5264-c3c0-d822-9e6f614ea93b@gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/e2395cdbe83adc50ac03dd17474ee88c5a97359a

Modified Files
--------------
src/backend/commands/tablecmds.c        | 26 +++++++++++++++-----------
src/test/regress/expected/create_am.out |  3 +++
src/test/regress/sql/create_am.sql      |  3 +++
3 files changed, 21 insertions(+), 11 deletions(-)