This change is part of this proposal:
https://discourse.llvm.org/t/rfc-all-the-math-intrinsics/78294
- `VecFuncs.def`: define intrinsic to sleef/armpl mapping
- `LegalizerHelper.cpp`: add missing fewerElementsVector handling for
the new atan2 intrinsic
- `AArch64ISelLowering.cpp`: Add arch64 specializations for lowering
like neon instructions
- `AArch64LegalizerInfo.cpp`: Legalize atan2.
Part 5 for Implement the atan2 HLSL Function #70096.