[AVR] Improve inline rotate/shift expansions

These expansions were rather inefficient and were done with more code
than necessary. This change optimizes them to use expansions more
similar to GCC. The code size is the same (when optimizing for code
size) but somehow LLVM reorders blocks in a non-optimal way. Still, this
should be an improvement with a reduction in code size of around 0.12%
(when building compiler-rt).

Differential Revision: https://reviews.llvm.org/D86418