AMDGPU: Record SGPR spills when restoring too
It's possible in some cases to have a restore present
without a corresponding spill. Due to an apparent bug
in D54366 https://reviews.llvm.org/D54366, only the
restore for a register was emitted. It's probably
always a bug for this to happen, but due to how SGPR
spilling is implemented, this makes the issues appear
worse than it is.