IFN: Implement IFN_VEC_SET for ARRAY_REF with VIEW_CONVERT_EXPR

IFN: Implement IFN_VEC_SET for ARRAY_REF with VIEW_CONVERT_EXPR

This patch enables transformation from ARRAY_REF(VIEW_CONVERT_EXPR) to
VEC_SET internal function in gimple-isel pass if target supports
vec_set with variable index by checking can_vec_set_var_idx_p.

gcc/ChangeLog:

2020-09-27  Xionghu Luo  <luoxhu@linux.ibm.com>

	* gimple-isel.cc (gimple_expand_vec_set_expr): New function.
	(gimple_expand_vec_cond_exprs): Rename to ...
	(gimple_expand_vec_exprs): ... this and call
	gimple_expand_vec_set_expr.
	* internal-fn.c (vec_set_direct): New define.
	(expand_vec_set_optab_fn): New function.
	(direct_vec_set_optab_supported_p): New define.
	* internal-fn.def (VEC_SET): New DEF_INTERNAL_OPTAB_FN.
	* optabs.c (can_vec_set_var_idx_p): New function.
	* optabs.h (can_vec_set_var_idx_p): New declaration.