Add support for (undocumented) HLSL 16-bit bit-cast ops (#1528)

As of SM 6.2, the dxc compiler added support for a set of 16-bit bit-cast operations to mirror the `asuint`, `asfloat`, and `asint` operations that were provided for 32-bit scalar types. These operations are not publicly documented, so we didn't think to add them.

It should be noted that there was already a similar operation in HLSL, called `f32tof16`, that took as input a `float` and then packed a half-precision version of it into the low bits of a `uint`. The problem is that using that operation for `half`->`uint16_t` conversion required a round trip through a `float`, and downstream compilers seemingly can't optimize away that conversion.

This change adds the new operations along with a test that tries to make use of them to ensure the results are what is expected. There are enough cases to cover that I had to write the test in a way where each thread only writes out a subset of the required output.

There are two other changes here are that are not directly related to the main feature:

First, it seems like the `[__forceInlineEarly]` attribute on some of these overloads interacts poorly with generics, and results in an `IRVectorType` appearing at local scope in the output code. That is semantically reasonable given our IR model, but it would ideally be something that gets eliminated as a result of deduplication of types. For now I've introduced a slight hack to make types always get inlined into their use sites during emission, which should handle the case of locally-defined types. I'm not 100% happy with that solution, but it seemed better than introducing a bunch of unrelated fixes into this PR.

Second, the way that conversion operations were being declared for matrix types seems to have been incorrect: we had a single *explicit* initializer added to matrix types via an `extension` that allowed them to be initialized from other matrix types with the same size and *any* element type. In order to support implicit conversions of matrix types, I cribbed the code we were already using to introduce implicit conversion operations for vector types.

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