RISC-V: Remove redundant vec_duplicate pattern

Currently, VLS and VLA patterns are different.
VLA is define_expand
VLS is define_insn_and_split

It makes no sense that they are different pattern format.
Merge them into same pattern (define_insn_and_split).
It can also be helpful for the future vv -> vx fwprop optimization.

gcc/ChangeLog:

	* config/riscv/riscv-selftests.cc (run_broadcast_selftests): Adapt selftests.
	* config/riscv/vector.md (@vec_duplicate<mode>): Remove.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/autovec/pr111313.c: Adapt test.