rs6000: Fix predicate for const vector in sldoi_to_mov [PR109069]

As PR109069 shows, commit r12-6537-g080a06fcb076b3 which
introduces define_insn_and_split sldoi_to_mov adopts
easy_vector_constant for const vector of interest, but it's
wrong since predicate easy_vector_constant doesn't guarantee
each byte in the const vector is the same.  One counter
example is the const vector in pr109069-1.c.  This patch is
to introduce new predicate const_vector_each_byte_same to
ensure all bytes in the given const vector are the same by
considering both int and float, meanwhile for the constants
which don't meet easy_vector_constant we need to gen a move
instead of just a set, and uses VECTOR_MEM_ALTIVEC_OR_VSX_P
rather than VECTOR_UNIT_ALTIVEC_OR_VSX_P for V2DImode support
under VSX since vector long long type of vec_sld is guarded
under stanza vsx.

	PR target/109069

gcc/ChangeLog:

	* config/rs6000/altivec.md (sldoi_to_mov<mode>): Replace predicate
	easy_vector_constant with const_vector_each_byte_same, add
	handlings in preparation for !easy_vector_constant, and update
	VECTOR_UNIT_ALTIVEC_OR_VSX_P with VECTOR_MEM_ALTIVEC_OR_VSX_P.
	* config/rs6000/predicates.md (const_vector_each_byte_same): New
	predicate.

gcc/testsuite/ChangeLog:

	* gcc.target/powerpc/pr109069-1.c: New test.
	* gcc.target/powerpc/pr109069-2-run.c: New test.
	* gcc.target/powerpc/pr109069-2.c: New test.
	* gcc.target/powerpc/pr109069-2.h: New test.