[AVR] Do not clear r0 at interrupt entry

There is no reason to do this: it's a scratch register and can therefore
hold any arbitrary value. And because it is in an interrupt, this code
is performance critical so it should be as short as possible.

I believe r0 was cleared because of the following:

 1. There used to be a bug that the cleared register was r0, not r1 as
    it should have been.
 2. This was fixed in https://reviews.llvm.org/D99467, but left the code
    to clear r0.

This patch completes D99467 by removing the `clr r0` instruction.

Differential Revision: https://reviews.llvm.org/D116756