AMDGPU/SI: Make sure MIMG descriptors and samplers stay in SGPRs

Summary:
It's possible to have resource descriptors and samplers stored in
VGPRs, either by a VMEM instruction or in the case of samplers,
floating-point calculations.  When this happens, we need to use
v_readfirstlane to copy these values back to sgprs.

Reviewers: mareko, arsenm

Subscribers: arsenm, llvm-commits

Differential Revision: http://reviews.llvm.org/D17102

llvm-svn: 260599