AMDGPU: Fix assert on aggregate type kernel arguments

Just fix the crash for now by not doing the optimization since
figuring out how to properly convert the bits for an arbitrary
struct is a pain.

Also fix a crash when there is only an empty struct argument.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@335827 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed