Revert "R600/SI: Add support for global atomic add"

This reverts commit r218254.

The global_atomics.ll test fails with asserts disabled.  For some reason,
the compiler fails to produce the atomic no return variants.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218257 91177308-0d34-0410-b5e6-96231b3b80d8