[ConstantRange] Make getEquivalentICmp smarter

This change teaches getEquivalentICmp to be smarter about generating
ICMP_NE and ICMP_EQ predicates.

An earlier version of this change was landed as rL283057 which had a
use-after-free bug.  This new version has a fix for that bug, and a (C++
unittests/) test case that would have triggered it rL283057.

llvm-svn: 283078