RISC-V: Extract vector duplicate for expand_const_vector [NFC]

RISC-V: Extract vector 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 vector duplicate
will be extracted into expand_const_vector_duplicate, and then
expand_const_vector_duplicate_repeating and
expand_const_vector_duplicate_default for the underlying function.

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

gcc/ChangeLog:

	* config/riscv/riscv-v.cc (expand_const_vector_duplicate_repeating):
	Add new func to take care of vector duplicate with repeating.
	(expand_const_vector_duplicate_default): Add new func to take
	care of default const vector duplicate.
	(expand_const_vector_duplicate): Add new func to take care
	of all const vector duplicate.
	(expand_const_vector): Extract const vector duplicate into
	separated function.

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