rs6000: Support [u]mul<mode>3_highpart for vector

rs6000: Support [u]mul<mode>3_highpart for vector

This patch is to make Power10 newly introduced vector
multiply high (part) instructions exploited in vectorized
loops, it renames existing define_insns as standard pattern
names.  It depends on that patch which enables vectorizer
to recog mul_highpart.

gcc/ChangeLog:

	* config/rs6000/vsx.md (mulhs_<mode>): Rename to...
	(smul<mode>3_highpart): ... this.
	(mulhu_<mode>): Rename to...
	(umul<mode>3_highpart): ... this.
	* config/rs6000/rs6000-builtin.def (MULHS_V2DI, MULHS_V4SI,
	MULHU_V2DI, MULHU_V4SI): Adjust.

gcc/testsuite/ChangeLog:

	* gcc.target/powerpc/mul-vectorize-3.c: New test.
	* gcc.target/powerpc/mul-vectorize-4.c: New test.