[RISCV] Support fixed-length vector i2fp/fp2i conversions

This patch extends the support for scalable-vector int->fp and fp->int
conversions by additionally handling fixed-length vectors.

The existing scalable-vector lowering re-expresses widening/narrowing by
x4+ conversions as standard nodes. The fixed-length vector support slots
in at "the end" of this process by lowering the now equally-sized and
widening/narrowing by x2 nodes to our custom VL versions.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D97374