Previously, AVRTargetLowering::LowerCall attempted to keep stack stores in order with chains. Perhaps this worked in the past, but it does not work now: it appears that the SelectionDAG legalization phase removes these chains. Therefore, I've removed these chains entirely to match X86 (which, similar to AVR, also prefers to use push instructions over stack-relative stores to set up a call frame). With this change, all the stack stores are in a somewhat reasonable order.
See also: https://reviews.llvm.org/D78581.
I ran my batch of compiler-rt tests locally and they still pass with this patch.
This patch is really a preparation for a different patch that attempts to optimize/fix the bug I originally tried to fix in D78581 but didn't do correctly.