AMDGPU: Rename OpenCL lowering pass to be R600 specific.

This pass is
  a) broken.
  b) r600 specific.

Fixing (a) is a bit more non-trivial, but fixing (b)
is easy. Move this pass to being R600 only for now.

This pass does pass all the unit tests, however clang
no longer generates code that looks like the unit test
input, so fixing the pass requires fixing the tests and
the pass as one, and checking it works with clang still.

Patch by Dave Airlie

llvm-svn: 332196