rs6000, Add vector shift double builtin support

rs6000, Add vector shift double builtin support

GCC maintainers:

The following patch adds support for the vector shift double builtins.

The patch has been compiled and tested on

  powerpc64le-unknown-linux-gnu (Power 8 LE)
  powerpc64le-unknown-linux-gnu (Power 9 LE)

and Mambo with no regression errors.

Please let me know if this patch is acceptable for the mainline branch.

Thanks.

                         Carl Love

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gcc/ChangeLog

	    2020-08-04  Carl Love  <cel@us.ibm.com>

	* config/rs6000/altivec.h (vec_sldb, vec_srdb): New defines.
	* config/rs6000/altivec.md (UNSPEC_SLDB, UNSPEC_SRDB): New.
	(SLDB_lr): New attribute.
	(VSHIFT_DBL_LR): New iterator.
	(vs<SLDB_lr>db_<mode>): New define_insn.
	* config/rs6000/rs6000-builtin.def (VSLDB_V16QI, VSLDB_V8HI,
	VSLDB_V4SI, VSLDB_V2DI, VSRDB_V16QI, VSRDB_V8HI, VSRDB_V4SI,
	VSRDB_V2DI): New BU_P10V_3 definitions.
	(SLDB, SRDB): New BU_P10_OVERLOAD_3 definitions.
	* config/rs6000/rs6000-call.c (P10_BUILTIN_VEC_SLDB,
	P10_BUILTIN_VEC_SRDB): New definitions.
	(rs6000_expand_ternop_builtin) [CODE_FOR_vsldb_v16qi,
	CODE_FOR_vsldb_v8hi, CODE_FOR_vsldb_v4si, CODE_FOR_vsldb_v2di,
	CODE_FOR_vsrdb_v16qi, CODE_FOR_vsrdb_v8hi, CODE_FOR_vsrdb_v4si,
	CODE_FOR_vsrdb_v2di]: Add clauses.
	* doc/extend.texi: Add description for vec_sldb and vec_srdb.

gcc/testsuite/ChangeLog

	    2020-08-04  Carl Love  <cel@us.ibm.com>

	* gcc.target/powerpc/vec-shift-double-runnable.c:  New test file.