[PATCH] [llvm] [Hexagon] Adding tests, TableGen entries, and code for emitting add opcode.

[Rafael Espíndola]

> Test quality is a good point.  What we've been doing internally to keep the test quality high is using them as our source-of-truth for correct encoding.  We thought unit-tests would be more of a library-centric approach rather than a toolchain-centric with files or calls out to other tools in the system.
>
> Do you think we can push up a few more of these unit tests in the next week or so and see how it shapes up or is this more of a blocking issue?  I think in the end you'll be happy with the quality.

I really don't want to have gtests for encoding in the long run. I
could see using them as a *short* term compromise, but It looks like
disassembler tests should do the job, why not use them?

Cheers,
Rafael