[SeparateConstOffsetFromGEP] Preserve metadata when splitting GEPs

Summary:
!amdgpu.uniform needs to be preserved for AMDGPU, otherwise bad things
happen.

Reviewers: arsenm, nhaehnle, jingyue, broune, majnemer, bjarke.roune, dblaikie

Subscribers: wdng, tpr, llvm-commits

Differential Revision: https://reviews.llvm.org/D42744

llvm-svn: 323907