[RISCV] Add a really basic cost model for SK_Splice.

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.

Reviewed By: ABataev

Differential Revision: https://reviews.llvm.org/D119316