Refactor predicate_{implied,refuted}_by_simple_clause.

Put the node-type-dependent operations into switches on nodeTag.
This should ease addition of new proof rules for other expression
node types.  There is no functional change, although some tests
are made in a different order than before.

Also, add a couple of new cross-checks in test_predtest.c.

James Coleman (part of a larger patch series)

Discussion: https://postgr.es/m/CAAaqYe8Bo4bf_i6qKj8KBsmHMYXhe3Xt6vOe3OBQnOaf3_XBWg@mail.gmail.com

Author: tglsfdc