[RISCV] Disable interleaving scalar loops in the loop vectorizer.

The loop vectorizer can interleave scalar loops even if it doesn't
vectorize them. I don't believe we intended to enable this when
we enabled interleaving for vector instructions.

Disable interleaving for VF=1 like X86 and AMDGPU already do. Test
lifted from AMDGPU.

Differential Revision: https://reviews.llvm.org/D115975