[AVR] Set R31R30 as clobbered after ADJCALLSTACKDOWN

In most cases, using R31R30 is fine because the call (which always
precedes ADJCALLSTACKDOWN) will clobber R31R30 anyway. However, in some
rare cases the register allocator might insert an instruction between
the call and the ADJCALLSTACKDOWN instruction and expect the register
pair to be live afterwards. I think this happens as a result of
rematerialization. Therefore, to fix this, the instruction needs to have
Defs set to R31R30.

Setting the Defs field does have the effect of making the instruction
look dead, which it certainly is not. This is fixed by setting
hasSideEffects to true.

Differential Revision: https://reviews.llvm.org/D97745