Choose ppc compare_exchange constant path for more operand values.

Choose ppc compare_exchange constant path for more operand values.

The implementation uses smaller code when the "expected" operand is a
small constant, but the implementation needlessly defined the set of
acceptable constants more narrowly than the ABI does.  Core PostgreSQL
and PGXN don't use the constant path at all, so this is future-proofing.
Back-patch to v13, where commit 30ee5d17c20dbb282a9952b3048d6ad52d56c371
introduced this code.

Reviewed by Tom Lane.  Reported by Christoph Berg.

Discussion: https://postgr.es/m/20201009092825.GD889580@msg.df7cb.de