Fix RTL simplifications of FFS, POPCOUNT and PARITY.

In 2011, the rtl.texi documentation was updated to reflect that the
modes of the RTX unary operations FFS, POPCOUNT and PARITY should
match those of their operands.  Unfortunately, some of the transformations
in simplify-rtx.cc predate this tightening of RTL semantics, and have
not (until now) been updated/fixed.  i.e. The POPCOUNT and PARITY
optimizations were "correct" when I added them back in 2007.

2023-02-27  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	* simplify-rtx.cc (simplify_unary_operation_1) <case FFS>: Avoid
	generating FFS with mismatched operand and result modes, by using
	an explicit SIGN_EXTEND/ZERO_EXTEND.
	<case POPCOUNT>: Likewise, for POPCOUNT of ZERO_EXTEND.
	<case PARITY>: Likewise, for PARITY of {ZERO,SIGN}_EXTEND.