[InstCombine] generalize 2 LSB of demanded bits for X*X

This is a follow-up suggested in D119060.
Instead of checking each of the bottom 2 bits individually,
we can check them together and handle the possibility that
we demand both together.

https://alive2.llvm.org/ce/z/C2ihC2

Differential Revision: https://reviews.llvm.org/D119139