[AMDGPU] Promote recursive loads from kernel argument to constant

Not clobbered pointer load chains are promoted to global now. That
is possible to promote these loads itself into constant address
space. Loaded pointers still need to point to global because we
need to be able to store into that pointer and because an actual
load from it may occur after a clobber.

Differential Revision: https://reviews.llvm.org/D119886