[RISCV] Add a fatal error if ISD::VSCALE is used with Zvl32b.

We convert VLEN to vscale by dividing by RVVBitsPerBlock which is
currently 64. This is only correct if VLEN is evenly divisible by
64. With only Zvl32b we can't assume that.

This patch adds a fatal_error to prevent generating code that may
be broken.

We probably need to look at how we size stack frame objects too.

Reviewed By: frasercrmck

Differential Revision: https://reviews.llvm.org/D118583