AMDGPU: Handle new address pattern in LowerKernelAttributes introduced by opaque pointers

Summary:
  With opaque pointer support, the "ptr" type is introduced and thus BitCast is not necessary in some cases.
This work takes care of this change, and recognizes the new address patterns to do appropriate optimizations.

Reviewers:
  arsenm

Differential Revision:
  https://reviews.llvm.org/D134596