[llvm] [ARM] Switch to soft promoting half types. (PR #80440)

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

I'm nervous about making type legalization disagree with getTypeToTransformTo().  For all other cases, it does agree, and some legalization code assumes it agrees.  Particularly for operations with f16 operands and non-f16 results.  The fact that this works at all seems to lean on very subtle invariants on conversions between f16 and non-f16 values.  And really, there's no reason it needs to disagree; the reason you're making it work this way is mostly just to avoid modifying existing code.

That said, I can't find anything specific that breaks.

https://github.com/llvm/llvm-project/pull/80440