[AMDGPU] Fix DPP sequence in atomic optimizer.

[AMDGPU] Fix DPP sequence in atomic optimizer.

This commit fixes the DPP sequence in the atomic optimizer (which was
previously missing the row_shr:3 step), and works around a read_register
exec bug by using a ballot instead.

Differential Revision: https://reviews.llvm.org/D57737

llvm-svn: 353703