[AArch64][SVE] Implement floating-point conversion intrinsics

Summary:
Adds intrinsics for the following:
  - fcvt
  - fcvtzs & fcvtzu
  - scvtf & ucvtf
  - fcvtlt, fcvtnt
  - fcvtx & fcvtxnt

Reviewers: huntergr, sdesmalen, dancgr, mgudim, efriedma

Reviewed By: sdesmalen

Subscribers: tschuett, kristof.beyls, hiraditya, rkruppe, psnobl, cameron.mcinally, cfe-commits, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D70180