Make the sqrt intrinsic return undef for a negative input.

As discussed here:
http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20140609/220598.html

And again here:
http://lists.cs.uiuc.edu/pipermail/llvmdev/2014-September/077168.html

The sqrt of a negative number when using the llvm intrinsic is undefined.
We should return undef rather than 0.0 to match the definition in the LLVM IR lang ref.

This change should not affect any code that isn't using "no-nans-fp-math";
ie, no-nans is a requirement for generating the llvm intrinsic in place of a sqrt function call.

Unfortunately, the behavior introduced by this patch will not match current gcc, xlc, icc, and
possibly other compilers. The current clang/llvm behavior of returning 0.0 doesn't either.
We knowingly approve of this difference with the other compilers in an attempt to flag code
that is invoking undefined behavior.

A front-end warning should also try to convince the user that the program will fail:
http://llvm.org/bugs/show_bug.cgi?id=21093

Differential Revision: http://reviews.llvm.org/D5527