EmitC: Allow casts between opaque and float types

[TinaAMD]

I can unfortunately not really test this since there is not type conversion from float to an opaque type.
We might want some made-for-testing-purposes opaque type conversions (in a separate PR) to ensure that opaque type conversions are handled properly in all upstream lowerings.

[TinaAMD]

Allowed opaque types no in more places and added tests to the CastOp to make sure the verifier is fine with the opaque type casts.
It is still enforced that if you convert an opaque type to an array, that needs to be an array ref. Should that restriction stay as-is?

[cferry-AMD]

It is still enforced that if you convert an opaque type to an array, that needs to be an array ref. Should that restriction stay as-is?

I would tend to say yes. Any cast into an array must be a reference cast from some other lvalue, otherwise as far as I know, that's illegal (I have a doubt on explicit casts of initializer lists though, that are not lvalues, but these are currently not supported). The best we can do if the type is opaque is to skip the lvalue-ness check.