AMDGPU: Select s_buffer_load_dword with a non-constant SGPR offset

Summary:
Apps that benefit:
- alien isolation
- bioshock infinite
- civilization: beyond earth
- company of heroes 2
- dirt showdown
- dota 2
- F1 2015
- grid autosport
- hitman
- legend of grimrock
- serious sam 3: bfe
- shadow warrior
- talos principle
- total war: warhammer
- UE4 demos: effects cave, elemental, sun temple

Reviewers: arsenm, nhaehnle

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

Differential Revision: https://reviews.llvm.org/D38914

llvm-svn: 317038