[AVR] Remove the earlyclobber flag from LDDWRdYQ

Before I started maintaining the AVR backend, this instruction
never originally used to have an earlyclobber flag.

Some time afterwards (years ago), I must've added it back in, not realising that it
was left out for a reason.

This pseudo instrction exists solely to work around a long standing bug
in the register allocator.

Before this commit, the LDDWRdYQ pseudo was not actually working around
any bug. With the earlyclobber flag removed again, the LDDWRdYQ pseudo
now correctly works around PR13375 again.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@326774 91177308-0d34-0410-b5e6-96231b3b80d8