rs6000: Add pdepd and pextd

Add scalar instructions for parallel bit deposit and extract, with
built-in function support.

[gcc]

2020-05-11  Kelvin Nilsen  <kelvin@gcc.gnu.org>

	* config/rs6000/rs6000-builtin.def (__builtin_pdepd): New built-in
	function.
	(__builtin_pextd): Likewise.
	* config/rs6000/rs6000.md (UNSPEC_PDEPD): New constant.
	(UNSPEC_PEXTD): Likewise.
	(pdepd): New insn.
	(pextd): Likewise.
	* doc/extend.texi (Basic PowerPC Built-in Functions Available for
	a Future Architecture): Add descriptions of __builtin_pdepd and
	__builtin_pextd functions.

[gcc/testsuite]

2020-05-11  Kelvin Nilsen  <kelvin@gcc.gnu.org>

	* gcc.target/powerpc/pdep-0.c: New.
	* gcc.target/powerpc/pdep-1.c: New.
	* gcc.target/powerpc/pextd-0.c: New.
	* gcc.target/powerpc/pextd-1.c: New.