match.pd: Simplify popcount/parity of bswap/rotate.

match.pd: Simplify popcount/parity of bswap/rotate.

This is the latest iteration of my patch from August 2020
https://gcc.gnu.org/pipermail/gcc-patches/2020-August/552391.html
incorperating feedback and suggestions from reviewers.

This patch to match.pd optimizes away bit permutation operations,
specifically bswap and rotate, in calls to popcount and parity.

2023-05-11  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	* match.pd <popcount optimizations>: Simplify popcount(bswap(x))
	as popcount(x).  Simplify popcount(rotate(x,y)) as popcount(x).
	<parity optimizations>:  Simplify parity(bswap(x)) as parity(x).
	Simplify parity(rotate(x,y)) as parity(x).

gcc/testsuite/ChangeLog
	* gcc.dg/fold-parity-6.c: New test.
	* gcc.dg/fold-parity-7.c: Likewise.
	* gcc.dg/fold-popcount-6.c: Likewise.
	* gcc.dg/fold-popcount-7.c: Likewise.