StackID is a property of a stack slot that allows a target to have
multiple spaces for spilling into. AMDGPU uses it to allow SGPRs to be
spilled into a VGPR, instead of using a real stack slot.
This commit fixes StackSlotColoring to ensure that two stack objects
with different StackID really don't share a slot, even if they are
colored out of order, and that the StackID gets set up in the colored
frame ids for later use.
Change-Id: I6a4d88ad7739d5d0550162b907e394b57fb3e437