AMDGPU: Refactor kernel argument lowering

Summary:
The main challenge in lowering kernel arguments for AMDGPU is determing the
memory type of the argument.  The generic calling convention code assumes
that only legal register types can be stored in memory, but this is not the
case for AMDGPU.

This consolidates all the logic AMDGPU uses for deducing memory types into a single
function.  This will make it much easier to support different ABIs in the future.

Reviewers: arsenm

Subscribers: arsenm, wdng, nhaehnle, llvm-commits, yaxunl

Differential Revision: https://reviews.llvm.org/D24614

llvm-svn: 281781