Replace the crude CSR spilling for GC transitions with something more sophisticated.

[pgavlin]

Codegen for callee-save register spilling is currently pretty crude: when lowering GC_TRANSITION_RA, we simply smash the register mask with a new mask that indicates that none of the GPRs that would otherwise have been saved are actually saved, which forces spill/restore generation later on down the road. It would be better to instead have the GC strategy inform the RewriteStatepoints pass that all GC pointers that are live across a GC transition must be spilled.

[AndyAyersMS]

I forget where we ended up on this, but if we don't spill all the callee saves in the prolog, then at pinvoke call sites we must spill/restore both live and dead callee saves.

In other words the GC must be able to find the values of all callee saves, live or not, in the managed frame, it can't look upstream into the unmanaged frames. So by the time we get to a pinvoke call site all caller values must have been spilled, as well as any new uses.