[AMDGPU] Promote generic pointer kernel arguments into global

[AMDGPU] Promote generic pointer kernel arguments into global

The new pass walks kernel's pointer arguments, then loads from them.
If a loaded value is a pointer and loaded pointer is unmodified in
the kernel before the load, then promote loaded pointer to global.
Then recursively continue.

Differential Revision: https://reviews.llvm.org/D111464