AMDGPU currently only supports direct calls, but at lower optimisation levels it
fails to lower statically direct calls which appear indirect due to a bitcast.
Add a pass to visit all CallSites and use CallPromotionUtils to "devirtualize"
calls.
Differential Revision: https://reviews.llvm.org/D52741
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@345382 91177308-0d34-0410-b5e6-96231b3b80d8
