[InstCombine] (ShiftValC >> Y) >s -1/<s 0 --> Y != 0/==0

We can do folds (ShiftValC >> Y) >s -1 --> Y != 0 and
(ShiftValC >> Y) <s 0 --> Y == 0, with ShiftValC < 0.

Alive2: https://alive2.llvm.org/ce/z/-PRHfD

Reviewed By: spatel

Differential Revision: https://reviews.llvm.org/D129726