AMDGPU: Start interpreting byref on kernel arguments

These are treated identically to value aggregates placed in the kernel
argument list. A %struct.foo or %struct.foo addrspace(4)*
byref(sizeof(%struct.foo)) align(alignof(%struct.foo)) argument should
produce the same offsets and argument metadata.

This handles all 3 kernel ABI implementations, and the two HSA
metadata emission paths.