3.6.1 patch nomination: r230564 - only propagate equality comparisons of FP values that we are certain are non-zero

[Simon Byrne]

I would like to nominate patch r230564 for backporting to the 3.6
branch. As it stands currently, the gvn pass in 3.6.0 can sometimes
result in invalid changes to the sign of floating point zeros: see,
for example,
http://llvm.org/bugs/show_bug.cgi?id=22823
and the related Julia thread,
https://github.com/JuliaLang/julia/issues/10377

Thank you,
Simon