[AMDGPU][SIFrameLowering] Mark VGPR used for AGPR spills as reserved

Presently, there is an issue on MI100 (and probably other architecture) where the VGPR used for AGPR copies clobbers VGPR used for AGPR spill. AFAICT this is because in processFunctionBeforeFrameIndicesReplaced we think the VGPR register for AGPR spill is unused. This patch aims to correct that. This is a WIP while I work out issues with producing a good test. For now, I'm curious if this is generally a good / bad idea.

Differential Revision: https://reviews.llvm.org/D139673