[GlobalISel] Limit shift_of_shifted_logic_chain to non-zero folds

After D157690 we are seeing some crashes from Global ISel, which seem to be
related to the shift_of_shifted_logic_chain combine that can remove too many
instructions if the shift amount is zero.

This limits the fold to non-zero shifts, under the assumption that it is better
in that case to fold away the shift to a COPY.

Differential Revision: https://reviews.llvm.org/D158596