RISC-V: Refactor the testcases for bswap16-0

RISC-V: Refactor the testcases for bswap16-0

This patch would like to refactor the testcases of bswap16-0
after sorts of optimization option passing to testcase.  To
fits the big lmul like m8 for asm dump check.

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

It is test only patch and obvious up to a point, will commit it
directly if no comments in next 48H.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/autovec/unop/bswap16-0.c: Update
	the vector register RE to cover v10 - v31.

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