Add `TYP_HALF` to the JIT.

[anthonycanino]

[jkotas]

What makes HALF so special that the VM needs to know about it? If the JIT needs to special-handle it, it should be able to do that without VM involvement.

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[anthonycanino]

You are correct --- it is only the JIT that needs to be updated. I misnamed based on earlier assumptions.

[JulieLeeMSFT]

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