[AVR] Expand large shifts early in IR

This patch makes sure shift instructions such as this one:

    %result = shl i32 %n, %amount

are expanded just before the IR to SelectionDAG conversion to a loop so
that calls to non-existing library functions such as __ashlsi3 are
avoided. The generated code is currently pretty bad but there's a lot of
room for improvement: the shift itself can be done in just four
instructions.

Differential Revision: https://reviews.llvm.org/D96677