RISC-V: Handle 0.0 floating point pattern costing to match const_vector expander

RISC-V: Handle 0.0 floating point pattern costing to match const_vector expander

The comment previously here stated that the Wc0/Wc1 cases are handled by
the vi constraint but that is not true for the 0.0 Wc0 case.

gcc/ChangeLog:

	* config/riscv/riscv-v.h (valid_vec_immediate_p): Add new helper.
	* config/riscv/riscv-v.cc (valid_vec_immediate_p): Ditto.
	(expand_const_vector): Use new helper.
	* config/riscv/riscv.cc (riscv_const_insns): Handle 0.0 floating-point
	case.

Signed-off-by: Patrick O'Neill <patrick@rivosinc.com>