[RISCV] Support fixed-length vector FP_ROUND & FP_EXTEND

This patch extends the support for vector FP_ROUND and FP_EXTEND by
including support for fixed-length vector types. Since fixed-length
vectors use "VL" nodes and scalable vectors can use the standard nodes,
there is slightly more to do in the fixed-length case. A helper function
was introduced to try and reduce the divergent paths. It is expected
that this function will similarly come in useful for lowering the
int-to-fp and fp-to-int operations for fixed-length vectors.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D97301