[llvm] [LoongArch] Pass 'half' in the lower 16 bits of an f32 value with F/D ABI (PR #109368)

[via llvm-commits]

heiher wrote:

**Update**: The design team behind the LoongArch ISA currently has **no plans** to add fp16 hardware extensions. Instead, they are leaning towards leveraging GPU and other hardware acceleration modules for fp16 computations. As a result, software calling conventions will continue to pass fp16 data using general-purpose registers.

I'll be reopening PR #109093 to ensure that fp16 support is functional on the release/19.x.

Thanks, everyone!

https://github.com/llvm/llvm-project/pull/109368