rs6000: Add vclrlb and vclrrb

Add new vector instructions to clear leftmost and rightmost bytes.

[gcc]

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

	* config/rs6000/altivec.h (vec_clrl): New #define.
	(vec_clrr): Likewise.
	* config/rs6000/altivec.md (UNSPEC_VCLRLB): New constant.
	(UNSPEC_VCLRRB): Likewise.
	(vclrlb): New insn.
	(vclrrb): Likewise.
	* config/rs6000/rs6000-builtin.def (__builtin_altivec_vclrlb): New
	built-in function.
	(__builtin_altivec_vclrrb): Likewise.
	(__builtin_vec_clrl): New overloaded built-in function.
	(__builtin_vec_clrr): Likewise.
	* config/rs6000/rs6000-call.c (altivec_overloaded_builtins):
	Define overloaded forms of __builtin_vec_clrl and
	__builtin_vec_clrr.
	* doc/extend.texi (PowerPC AltiVec Built-in Functions Available
	for a Future Architecture): Add descriptions of vec_clrl and
	vec_clrr.

[gcc/testsuite]

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

	* gcc.target/powerpc/vec-clrl-0.c: New.
	* gcc.target/powerpc/vec-clrl-1.c: New.
	* gcc.target/powerpc/vec-clrr-0.c: New.
	* gcc.target/powerpc/vec-clrr-1.c: New.