[RISCV] Support the scalable-vector fadd reduction intrinsic

This patch adds support for both the fadd reduction intrinsic, in both
the ordered and unordered modes.

The fmin and fmax intrinsics are not currently supported due to a
discrepancy between the LLVM semantics and the RVV ISA behaviour with
regards to signaling NaNs. This behaviour is likely fixed in version 2.3
of the RISC-V F/D/Q extension, but until then the intrinsics can be left
unsupported.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D95870