[AArch64][SVE/NEON] Add support for FROUNDEVEN for both NEON and fixed length SVE

Previously NEON used a target specific intrinsic for frintn, given that
the FROUNDEVEN ISD node now exists, move over to that instead and add
codegen support for that node for both NEON and fixed length SVE.

Differential Revision: https://reviews.llvm.org/D98487