[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
