This patch adds an initial skeleton for `SelectionDAG::computeKnownFPClass`.
The initial version includes:
- DemandedElts wrapper and max depth early-out
- `ConstantFPSDNode` and `BUILD_VECTOR` handling
- `TargetLowering::computeKnownFPClassForTargetNode` virtual hook for backend extensions
Initial test coverage for constant scalars, BUILD_VECTOR, and max depth
early-out is added in `AArch64SelectionDAGTest.cpp`.
closes#175571
aa22fca59a