[RISCV] Add support for fixed vector floating point setcc.

This is annoying because the condition code legalization belongs
to LegalizeDAG, but our custom handler runs in Legalize vector ops
which occurs earlier.

This adds some of the mask binary operations so that we can combine
multiple compares that we need for expansion.

I've also fixed up RISCVISelDAGToDAG.cpp to handle copies of masks.

This patch contains a subset of the integer setcc patch as well.
That patch is dependent on the integer binary ops patch. I'll rebase
based on what order the patches go in.

Reviewed By: frasercrmck

Differential Revision: https://reviews.llvm.org/D96567