RISC-V: Add duplicate vector support.

RISC-V: Add duplicate vector support.

gcc/ChangeLog:

	* config/riscv/constraints.md (Wdm): New constraint.
	* config/riscv/predicates.md (direct_broadcast_operand): New predicate.
	* config/riscv/riscv-protos.h (RVV_VLMAX): New macro.
	(emit_pred_op): Refine function.
	* config/riscv/riscv-selftests.cc (run_const_vector_selftests): New function.
	(run_broadcast_selftests): Ditto.
	(BROADCAST_TEST): New tests.
	(riscv_run_selftests): More tests.
	* config/riscv/riscv-v.cc (emit_pred_move): Refine function.
	(emit_vlmax_vsetvl): Ditto.
	(emit_pred_op): Ditto.
	(expand_const_vector): New function.
	(legitimize_move): Add constant vector support.
	* config/riscv/riscv.cc (riscv_print_operand): New asm print rule for const vector.
	* config/riscv/riscv.h (X0_REGNUM): New macro.
	* config/riscv/vector-iterators.md: New attribute.
	* config/riscv/vector.md (vec_duplicate<mode>): New pattern.
	(@pred_broadcast<mode>): New pattern.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/base/dup-1.c: New test.
	* gcc.target/riscv/rvv/base/dup-2.c: New test.