[llvm] [AVR] Support stack realignment (PR #187129)

[Ben Shi via llvm-commits]

benshi001 wrote:

For your older solution: https://github.com/Patryk27/llvm-project/tree/avr-alloca-align-old.

Is it possible to use `R3:R2` on AVR and `R19:R18` on AVRTiny as `SPReg` ?  Since

1. They are callee-saved registers.
2. They are not used for parameter passing.

It may generates less efficient assembly, but the older solution is much more simple than current one.

https://github.com/llvm/llvm-project/pull/187129