[LVI] NFC. Make getValueFromCondition return LVILatticeValue instead of changing reference argument

Instead of returning bool and setting LVILatticeValue reference argument return LVILattice value. Use overdefined value to denote the case when we didn't gather any information from the condition.

This change was separated from the review "[LVI] Handle conditions in the form of (cond1 && cond2)" (https://reviews.llvm.org/D23200#inline-199531). Once getValueFromCondition returns LVILatticeValue we can cache the result in Visited map.

llvm-svn: 278224