[SCCP] Propagate inequalities

Teach SCCP to create notconstant lattice values from inequality
assumes and nonnull metadata, and update getConstant() to make
use of them. Additionally isOverdefined() needs to be changed to
consider notconstant an overdefined value.

Handling inequality branches is delayed until our branch on undef
story in other passes has been improved.

Differential Revision: https://reviews.llvm.org/D83643