rs6000: Change rs6000_expand_vector_set param

rs6000: Change rs6000_expand_vector_set param

rs6000_expand_vector_set could accept insert either to constant position
or variable position, so change the operand to reg_or_cint_operand.

gcc/ChangeLog:

2020-11-27  Xionghu Luo  <luoxhu@linux.ibm.com>

	* config/rs6000/rs6000-call.c (altivec_expand_vec_set_builtin):
	Change call param 2 from type int to rtx.
	* config/rs6000/rs6000-protos.h (rs6000_expand_vector_set):
	Likewise.
	* config/rs6000/rs6000.c (rs6000_expand_vector_init):
	Change call param 2 from type int to rtx.
	(rs6000_expand_vector_set): Likewise.
	* config/rs6000/vector.md (vec_set<mode>): Support both constant
	and variable index vec_set.