AMDGPU/R600: Move int_r600_store_stream_output to the public intrinsic file

Summary:
The TableGen'd GlobalISel instruction selector assumes all intrinsics are in
the public Intrinsic:: namespace.

Reviewers: jvesely, nhaehnle

Reviewed By: jvesely, nhaehnle

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

Differential Revision: https://reviews.llvm.org/D45989

llvm-svn: 330866