[AMDGPU] Use inreg for hint to preload kernel arguments

This patch is the first in a series that adds support for pre-loading
kernel arguments into SGPRs. The command-line argument
'amdgpu-kernarg-preload-count' is used to specify the number of
arguments sequentially from the first that we should attempt to preload,
the default is 0.

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D156852