[HLSL] Shore up floating point conversions (#90222)

This PR fixes bugs in HLSL floating conversions. HLSL always has `half`,
`float` and `double` types, which promote in the order:

`half`->`float`->`double`

and convert in the order:

`double`->`float`->`half`

As with other conversions in C++, promotions are preferred over
conversions.

We do have floating conversions documented in the draft language
specification (https://microsoft.github.io/hlsl-specs/specs/hlsl.pdf
[Conv.rank.float]) although the exact language is still in flux
(https://github.com/microsoft/hlsl-specs/pull/206).

Resolves #81047