[SPIRV] For 16 bit isnan generate 16 bit OpIsNan SPIRV Op.

[spall]

Currently the operation is extended to the 32 bit version.

1. For 16 bit isnan we want to always generate the 16 bit OpIsNan
2. Add tests showing we generate the 16 bit op

[bogner]

Probably just needs a test once the frontend has the correct overload.