Add an ID field to StackObjects

Add an ID field to StackObjects

On AMDGPU SGPR spills are really spilled to another register.
The spiller creates the spills to new frame index objects,
which is used as a placeholder.

This will eventually be replaced with a reference to a position
in a VGPR to write to and the frame index deleted. It is
most likely not a real stack location that can be shared
with another stack object.

This is a problem when StackSlotColoring decides it should
combine a frame index used for a normal VGPR spill with
a real stack location and a frame index used for an SGPR.

Add an ID field so that StackSlotColoring has a way
of knowing the different frame index types are
incompatible.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@308673 91177308-0d34-0410-b5e6-96231b3b80d8