AMDGPU/SI: Insert wait states required after v_readfirstlane on SI

Summary:
We will be able to handle this case much better once the hazard recognizer
is finished, but this conservative implementation  fixes a hang with the piglit
test:

spec/arb_arrays_of_arrays/execution/sampler/fs-nested-struct-arrays-nonconst-nested-arra

Reviewers: arsenm, nhaehnle

Subscribers: arsenm, llvm-commits

Differential Revision: http://reviews.llvm.org/D18988

llvm-svn: 266105