While testing scalable vectors I found that if we generate a
vector splice intrinsic and run the code through the loop unroller,
we'll crash due to an invalid cost.
This adds a basic cost based on the 2 slide instructions used by the
lowering in D119303.
We probably need to factor LMUL into this, but that's true for
arithmetic instructions too. So I've ignored for the moment.
clang-format not found in user’s local PATH; not linting file.