[Bug target/84209] [avr] Don't split SP in split2

[gjl at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84209

--- Comment #1 from Georg-Johann Lay <gjl at gcc dot gnu.org> ---
Created attachment 43338
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=43338&action=edit
Proposed patch against v7