AMDGPU: Be explicit about whether the high-word in SI_PC_ADD_REL_OFFSET is 0

Summary:
Instead of encoding a high-word of 0 using a fake TargetGlobalAddress,
just use a literal target constant. This simplifies some subsequent changes.

The generated assembly is now more explicit about the kind of relocation
that is to be used.

Change-Id: I066835202d23b5941fa7a358eb4b89e9b71ab6f8

Reviewers: arsenm, rampitec

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D61491

llvm-svn: 363516