[LV][RISCV] Don't interleave scalable vector loops

It's less clear with scalable vectors than fixed length vectors that
interleaving exposes more ILP, as scalable vectors can be thought of a
sort of hardware form of interleaving, especially with larger LMULs.
This also addresses the unexpected additional unrolling that occurs when
using larger LMULs in the loop vectorizer.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D144485