[RISCV] Fix assertion in lowerEXTRACT_SUBVECTOR

This fixes a crash when lowering an extract_subvector like:

t0:v1i64 = extract_subvector t1:v2i64, 1

Whilst we never need a vslidedown with M1 on scalable vector types, we might
need to do it for v1i64/v1f64, since the smallest container type for it is
nxv1i64/nxv1f64.

The lowering code is still correct for this case, but the assertion was too
strict. The actual invariant we're relying on is that ContainerSubVecVT's LMUL
<= M1, not < M1. Hence why we handled v2i32 fine, because its container type
was nxv1i32 and MF2.