[AVR] Remove redundant dynalloca SP save/restore pass

I think this pass was previously used under the assumption that most
functions would not need a frame pointer and it would be more efficient
to store the old stack pointer in a regular register pair.

Unfortunately, right now we're forced to always reserve the Y register
as a frame pointer: whether or not this is needed is only known after
regsiter allocation at which point it doesn't make sense anymore to mark
it as non-reserved. Therefore, it makes sense to use the Y register to
store the old stack pointer in functions with dynamic allocas (with a
variable size or not in the entry block). Knowing this can make the code
around dynamic allocas a lot simpler: simply save/restore the frame
pointer.

This is especially relevant in functions that have a frame pointer
anyway (for example, because they have stack spills). The stack restore
in the epilogue will implicitly restore the old stack pointer, so there
is no need to store the old stack pointer separately. It even reduces
register pressure as a side effect.

Differential Revision: https://reviews.llvm.org/D97815