[RISCV] Create the correct mask type when lowering EXTRACT_VECTOR_ELT

This particular case was creating a `VMSET_VL` using the old
fixed-length type in order to pass a mask to other custom nodes
operating on the scalable container type. This kind of thing wasn't
caught for us; I only noticed when experimenting with odd-length
vectors, where it was trying to generate an invalid `v3i1` MVT.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D110420