[analyzer] ConditionBRVisitor: Remove GDM checking

Summary:
Removed the `GDM` checking what could prevent reports made by this visitor.
Now we rely on constraint changes instead.
(It reapplies 356318 with a feature from 356319 because build-bot failure.)

Reviewers: NoQ, george.karpenkov

Reviewed By: NoQ

Subscribers: cfe-commits, jdoerfert, gerazo, xazax.hun, baloghadamsoftware,
szepet, a.sidorin, mikhail.ramalho, Szelethus, donat.nagy, dkrupp

Tags: #clang

Differential Revision: https://reviews.llvm.org/D54811

llvm-svn: 356322