[AMDGPU] Avoid using divergent value in mubuf addr64 descriptor

Summary:
This fixes a problem where a load from global+idx generated incorrect
code on <=gfx7 when the index is divergent.

Subscribers: arsenm, kzhuravl, wdng, nhaehnle, yaxunl, dstuttard, t-tye, llvm-commits

Differential Revision: https://reviews.llvm.org/D47383

Change-Id: Ib4d177d6254b1dd3f8ec0203fdddec94bd8bc5ed
llvm-svn: 338779