rs6000: Add cntlzdm and cnttzdm

rs6000: Add cntlzdm and cnttzdm

Add support for new scalar instructions for counting leading or
trailing zeros under control of a bitmask.

[gcc]

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

	* config/rs6000/rs6000-builtin.def (__builtin_cntlzdm): New
	built-in function definition.
	(__builtin_cnttzdm): Likewise.
	* config/rs6000/rs6000.md (UNSPEC_CNTLZDM): New constant.
	(UNSPEC_CNTTZDM): Likewise.
	(cntlzdm): New insn.
	(cnttzdm): Likewise.
	* doc/extend.texi (Basic PowerPC Built-in Functions available for
	a Future Architecture): Add descriptions of __builtin_cntlzdm and
	__builtin_cnttzdm functions.

[gcc/testsuite]

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

	* gcc.target/powerpc/cntlzdm-0.c: New test.
	* gcc.target/powerpc/cntlzdm-1.c: New test.
	* gcc.target/powerpc/cnttzdm-0.c: New test.
	* gcc.target/powerpc/cnttzdm-1.c: New test.