RISC-V: Move optimization patterns into autovec-opt.md

Move all optimization patterns into autovec-opt.md to make organization
easier maintain.

gcc/ChangeLog:

	* config/riscv/autovec-opt.md (*<optab>not<mode>): Move to autovec-opt.md.
	(*n<optab><mode>): Ditto.
	* config/riscv/autovec.md (*<optab>not<mode>): Ditto.
	(*n<optab><mode>): Ditto.
	* config/riscv/vector.md: Ditto.