[analyzer][solver] Introduce reasoning for not equal to operator

Prior to this, the solver was only able to verify whether two symbols
are equal/unequal, only when constants were involved. This patch allows
the solver to work over ranges as well.

Reviewed By: steakhal, martong

Differential Revision: https://reviews.llvm.org/D106102

Patch by: @manas (Manas Gupta)