Partially revert D61491 "AMDGPU: Be explicit about whether the high-word in...

Partially revert D61491 "AMDGPU: Be explicit about whether the high-word in SI_PC_ADD_REL_OFFSET is 0"

Summary:
D61491 caused us to use relocs when they're not strictly necessary, to
refer to symbols in the text section. This is a pessimization and it's a
problem for some loaders that don't support relocs yet.

Reviewers: nhaehnle, arsenm, tpr

Subscribers: kzhuravl, jvesely, wdng, yaxunl, dstuttard, t-tye, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D65813

llvm-svn: 370667