only propagate equality comparisons of FP values that we are certain are non-zero

only propagate equality comparisons of FP values that we are certain are non-zero

This is a follow-on to r227491 which tightens the check for propagating FP
values. If a non-constant value happens to be a zero, we would hit the same
bug as before.

Bug noted and patch suggested by Eli Friedman.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@230564 91177308-0d34-0410-b5e6-96231b3b80d8