[RISCV] Optimize scalable frame setup when VLEN is precisely known

If we know the exact value of VLEN, the frame offset adjustment for scalable stack slots becomes a fixed constant. This avoids the need to read vlenb, and may allow the offset to be folded into the immediate field of an add/sub.

We could go further here, and fold the offset into a single larger frame adjustment - instead of having a separate scalable adjustment step - but that requires a bit more code reorganization. I may (or may not) return to that in a future patch.

Differential Revision: https://reviews.llvm.org/D137593