[RISCV][LSR] Add coverage for ICmpZero with scaled vscale values

Follow up to 3bc09c7d - remove a fixme I forgot to remove, and add test cases showing remaining work.

Note that scaled vscales show up in vectorized code from a couple of sources:
* Element types smaller than vector block size (i.e. everything under i64)
* Unrolling
* LMUL > 1

The largest scaling we can currently have is 256 (e8 in every possible vector register).  More practically useful scales are in the 2-16 range.