[AMDGPU] Add an AMDGPU specific atomic optimizer.

This commit adds a new IR level pass to the AMDGPU backend to perform
atomic optimizations. It works by:

- Running through a function and finding atomicrmw add/sub or uses of
  the atomic buffer intrinsics for add/sub.
- If all arguments except the value to be added/subtracted are uniform,
  record the value to be optimized.
- Run through the atomic operations we can optimize and, depending on
  whether the value is uniform/divergent use wavefront wide operations
  (DPP in the divergent case) to calculate the total amount to be
  atomically added/subtracted.
- Then let only a single lane of each wavefront perform the atomic
  operation, reducing the total number of atomic operations in flight.
- Lastly we recombine the result from the single lane to each lane of
  the wavefront, and calculate our individual lanes offset into the
  final result.

Differential Revision: https://reviews.llvm.org/D51969

llvm-svn: 343973