[AVR] Optimize 32-bit shift: move bytes around

This patch optimizes 32-bit constant shifts by renaming registers. This
is very effective as the compiler would otherwise need to do a lot of
single bit shift instructions. Instead, the registers are renamed at the
SSA level which means the register allocator will insert the necessary
mov instructions.

Unfortunately, the register allocator will insert some unnecessary movs
with the current code. This will be fixed in a later patch.

Differential Revision: https://reviews.llvm.org/D140570