[RISCV] Optimize scalable frame offset calculation when VLEN is precisely known

When we have a precisely known VLEN, we can replace runtime usage of VLENB with compile time constants. This converts offsets involving both fixed and scalable components into fixed offsets. The result is that we avoid the csr read of vlenb, and can often fold the multiply as well.

Differential Revision: https://reviews.llvm.org/D137591