Draft PR for in-progress work to accelerate System.Half with FP16 ISA.

Current work done:

1. Add a TYP_HALF to the .NET runtime, which is treated like a TYP_SIMDXX, but with some notable differences. Namely, a TYP_HALF is passed around via the xmm registers, and while it will pass a varTypeIsStruct test, it must be treated as a primitive in other places.

2. Accelerate System.Half operations with the TYP_HALF and some FP16 intrinsics. Not every possible function has been accelerated yet.

For discussion:

1. I have currently worked around some checks to make TYP_HALF behave like a struct and a primitive. It's very ad-hoc at the moment.

2. Much of the work to transform the named System.Half intrinsics into a sequence of intrinsic nodes is done in importcall.cpp and might want to be moved up into some of the gtNewSimdXX nodes.