Add vadd_vx unit test

Adding `vadd.vx`, keeping consistent with implementation for vmv and vadd_vi.
Tests have been verified to pass in Qemu.

Note: our current approach limits to one test case per sew/lmul pair.

Change-Id: Ibc744e989d5a92496ae63bc6c7802e2152b2e4e1
2 files changed