[AVR] Remove regalloc workaround for LDDWRdPtrQ

Background: https://github.com/avr-rust/rust-legacy-fork/issues/126

In short, this workaround was introduced to fix a "ran out of registers
during regalloc" issue. The root cause has since been fixed in
https://reviews.llvm.org/D54218 so this workaround can be removed.

There is one test that changes a little bit, removing a single
instruction. I also compiled compiler-rt before and after this patch but
didn't see a difference. So presumably the impact is very low. Still,
it's nice to be able to remove such a workaround.

Differential Revision: https://reviews.llvm.org/D117831