RISC-V: Allow all const_vec_duplicates as constants.

RISC-V: Allow all const_vec_duplicates as constants.

As we can always broadcast an integer constant to a vector register
allow them in riscv_const_insns.  We need as many instructions as
it takes to generate the constant and one vmv.vx.

gcc/ChangeLog:

	* config/riscv/riscv.cc (riscv_const_insns): Allow
	const_vec_duplicates.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/autovec/vmv-imm-fixed-rv32.c: Add vmv.v.x
	tests.
	* gcc.target/riscv/rvv/autovec/vmv-imm-fixed-rv64.c: Dito.
	* gcc.target/riscv/rvv/autovec/vmv-imm-run.c: Dito.
	* gcc.target/riscv/rvv/autovec/vmv-imm-rv32.c: Dito.
	* gcc.target/riscv/rvv/autovec/vmv-imm-rv64.c: Dito.
	* gcc.target/riscv/rvv/autovec/vmv-imm-template.h: Dito.