[AMDGPU] Transform __read_pipe_* and __write_pipe_*

When packet size equals packet align and is power of 2, transform
__read_pipe* and __write_pipe* to specialized library function.

Differential Revision: https://reviews.llvm.org/D36831


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@312598 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed