[PATCH 5/7] RISC-V: Move vector memcpy decision making to separate function [NFC]

This moves the code for deciding whether to generate a vectorized
memcpy, what vector mode to use and whether a loop is needed out of
riscv_vector::expand_block_move and into a new function
riscv_vector::use_stringop_p so that it can be reused for other string
operations.

gcc/ChangeLog:

	* config/riscv/riscv-string.cc (struct stringop_info): New.
	(expand_block_move): Move decision making code to...
	(use_vector_stringop_p): ...here.