PATCH for PR18000: wrong code at -Os and above on x86_64-linux-gnu

[Stepan Dyatkovskiy]

Hello,

The things starts to be wrong, as I assume (thanks to David Wiberg) in 
ScalarEvolution::getZeroExtendExpr

As I think the entire problem caused by an ancient typo in 
ScalarEvolution::isImpliedCond that is invoked inside (patch changes 
only one line).

For people how knows ScalarEvolution enough it would be easy to 
understand whether it was typo.

I also posted detailed explanations here:
http://llvm.org/bugs/show_bug.cgi?id=18000#c16

Thanks!

-Stepan.