[LibCallSimplifier] partly fix pow(x, 0.5) -> sqrt() transforms

As the first test shows, we could transform an llvm intrinsic which never sets errno 
into a libcall which could set errno (even though it's marked readnone?), so that's 
not ideal.

It's possible that we can also transform a libcall which could set errno to an
intrinsic given the fast-math-flags constraint, but that's deferred to determine
exactly which set of FMF are needed.

Differential Revision: https://reviews.llvm.org/D40150

llvm-svn: 318628