a bunch of half-related stuff

  - add f32<->f16 functions to skvx
  - add f32<->f16 x86 instructions to skvm::Assembler
  - add f32<->f16 ops to skvm,
    using the skvx functions in the interpreter

Still TODO:
    use the new x86 instructions in the JIT

(For now like in many other ways, the aarch64 JIT
continues to languish.  Will pick that back up one day.)

Change-Id: Ib8dc1ccdc75ecb23769ea4947d66d3ab22520f23
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/302942
Commit-Queue: Mike Klein <mtklein@google.com>
Reviewed-by: Herb Derby <herb@google.com>
6 files changed