AMDGPU/SI: Make sure llvm.amdgcn.implicitarg.ptr() is at least 4-byte aligned

Summary: This fixes some OpenCV tests that were broken by libclc commit r276443.

Reviewers: arsenm, jvesely

Subscribers: arsenm, wdng, llvm-commits

Differential Revision: https://reviews.llvm.org/D24051

llvm-svn: 280274