R600/SI: Initial support for assembler and inline assembly

This is currently considered experimental, but most of the more
commonly used instructions should work.

So far only SI has been extensively tested, CI and VI probably work too,
but may be buggy.  The current set of tests cases do not give complete
coverage, but I think it is sufficient for an experimental assembler.

See the documentation in R600Usage for more information.

llvm-svn: 234381