AMDGPU/GlobalISel: Implement custom kernel arg lowering

Avoid using allocateKernArg / AssignFn. We do not want any
of the type splitting properties of normal calling convention
lowering.

For now at least this exists alongside the IR argument lowering
pass. This is necessary to handle struct padding correctly while
some arguments are still skipped by the IR argument lowering
pass.

llvm-svn: 336373