RISC-V: Move helper functions above expand_const_vector

These subroutines will be used in expand_const_vector in a future patch.
Relocate so expand_const_vector can use them.

gcc/ChangeLog:

	* config/riscv/riscv-v.cc (expand_vector_init_insert_elems): Relocate.
	(expand_vector_init_trailing_same_elem): Ditto.

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