[GVN] don't propagate equality comparisons of FP zero (PR22376)

[GVN] don't propagate equality comparisons of FP zero (PR22376)

In http://reviews.llvm.org/D6911, we allowed GVN to propagate FP equalities
to allow some simple value range optimizations. But that introduced a bug
when comparing to -0.0 or 0.0: these compare equal even though they are not
bitwise identical.

This patch disallows propagating zero constants in equality comparisons.
Fixes: http://llvm.org/bugs/show_bug.cgi?id=22376
Differential Revision: http://reviews.llvm.org/D7257

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@227491 91177308-0d34-0410-b5e6-96231b3b80d8