[AArch64][SVE] Implement several floating-point arithmetic intrinsics

Summary:
Adds intrinsics for the following:
  - fabd, fadd, fsub & fsubr
  - fmul, fmulx, fdiv & fdivr
  - fmax, fmaxnm, fmin & fminnm
  - fscale & ftsmul

Reviewers: huntergr, sdesmalen, dancgr

Reviewed By: sdesmalen

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D69657