[RISCV] Add scalable vector fadd/fsub/fmul/fdiv ISel patterns

Original patch by @rogfer01.

This patch adds ISel patterns for the above operations to the
corresponding vector/vector and vector/scalar RVV instructions, as well
as extra patterns to match operand-swapped scalar/vector vfrsub and
vfrdiv.

Authored-by: Roger Ferrer Ibanez <rofirrim@gmail.com>
Co-Authored-by: Fraser Cormack <fraser@codeplay.com>

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D94408