The default `f16` lowering has some issues that result in incorrect
float behavior, so over time most targets have switched to use
`softPromoteHalfType`. Swap to soft promotion by default and add
overrides for SystemZ and AMDGPU, which are the two remaining backends
that still depend on this behavior.
All basic `f16` op tests now pass on all remaining experimental arches.
Fixes: https://github.com/llvm/llvm-project/issues/97981
Fixes: https://github.com/llvm/llvm-project/issues/97975
3920bc61ca