[llvm] [TargetLowering] Change the `softPromoteHalfType` default to `true` (PR #175149)

[Nikita Popov via llvm-commits]

nikic wrote:

One test failure on SystemZ, 154 on AMDGPU :) I'd suggest to opt-out those backends in this PR. Can then flip SystemZ (should be simple, only affects soft-float) and AMDGPU is going to need some more analysis.

https://github.com/llvm/llvm-project/pull/175149