[RISCVInsertVSETVLI] Default to VL=1 for scalar extracts

We were defaulting to VL=0 when we didn't otherwise have a vsetv
nearby. Instead, let's use VL=1. VL=0 is very much a cornercase
in hardware, and let's avoid if we can.

Differential Revision: https://reviews.llvm.org/D158015