RISC-V: Reorganize riscv-v.cc

This patch is just reorganizing the functions for the following patch.

I put rvv_builder and emit_* functions located before expand_const_vector
function since I will use them in expand_const_vector in the following patch.

gcc/ChangeLog:

	* config/riscv/riscv-v.cc (class rvv_builder): Reorganize functions.
	(rvv_builder::can_duplicate_repeating_sequence_p): Ditto.
	(rvv_builder::repeating_sequence_use_merge_profitable_p): Ditto.
	(rvv_builder::get_merged_repeating_sequence): Ditto.
	(rvv_builder::get_merge_scalar_mask): Ditto.
	(emit_scalar_move_insn): Ditto.
	(emit_vlmax_integer_move_insn): Ditto.
	(emit_nonvlmax_integer_move_insn): Ditto.
	(emit_vlmax_gather_insn): Ditto.
	(emit_vlmax_masked_gather_mu_insn): Ditto.
	(get_repeating_sequence_dup_machine_mode): Ditto.