[AMDGPU] Added checks for dpp_ctrl value

- Report error for invalid dpp_ctrl values.
- Changed the way it is reported, now the error will be emitted into
  asm and will work with release build as well.
- Added dpp_ctrl value verifier for codegen.
- Added symbolic constants for dpp_ctrl.

Differential Revision: https://reviews.llvm.org/D46565

llvm-svn: 331775