RISC-V: Extract vec_duplicate for expand_const_vector [NFC]

RISC-V: Extract vec_duplicate for expand_const_vector [NFC]

Consider the expand_const_vector is quit long (about 500 lines)
and complicated, we would like to extract the different case
into different functions.  For example, the const vec_duplicate
will be extracted into expand_const_vec_duplicate.

The below test suites are passed for this patch.
* The rv64gcv fully regression test.

gcc/ChangeLog:

	* config/riscv/riscv-v.cc (expand_const_vector): Extract
	const vec_duplicate into separated function.
	(expand_const_vec_duplicate): Add new func to take care
	of the const vec_duplicate.

Signed-off-by: Pan Li <pan2.li@intel.com>