[AVR] Optimize 32-bit shifts: shift by 4 bits

This uses a complicated shift sequence that avr-gcc also uses, but
extended to work over any number of bytes and in both directions
(logical shift left and logical shift right). Unfortunately it can't be
used for an arithmetic shift right: I've tried to come up with a
sequence but couldn't.

Differential Revision: https://reviews.llvm.org/D140571