Revert "[AMDGPU] For OS type AMDPAL, fixed scratch on compute shader"

This reverts commit 0daf86291d3aa04d3cc280cd0ef24abdb0174981.

It was causing an assert in test/CodeGen/AMDGPU/amdpal.ll only on a
release-with-asserts build. I will resubmit the change when I have fixed
that.

Change-Id: If270594eba27a7dc4076bdeab3fa8e6bfda3288a
llvm-svn: 328695