Revert "AMDGPU: Divergence-driven selection of scalar buffer load intrinsics"

This reverts commit r344696 for now (except for some test additions).

See https://bugs.freedesktop.org/show_bug.cgi?id=108611.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@346364 91177308-0d34-0410-b5e6-96231b3b80d8