[AVR] Only push and clear R1 in interrupts when necessary

R1 is a reserved register, but LLVM gives the APIs to know when it is
used or not. So this patch uses these APIs to only save/clear/restore R1
in interrupts when necessary.

The main issue here was getting inline assembly to work. One could argue
that this is the job of Clang, but for consistency I've made sure that
R1 is always usable in inline assembly even if that means clearing it
when it might not be needed.

Information on inline assembly in AVR can be found here:

https://www.nongnu.org/avr-libc/user-manual/inline_asm.html#asm_code

Essentially, this seems to suggest that r1 can be freely used in avr-gcc
inline assembly, even without specifying it as an input operand.

Differential Revision: https://reviews.llvm.org/D117426