SimplifyLibCalls: When emitting an overloaded fp function check that it's available.

The existing code missed some edge cases when e.g. we're going to emit sqrtf but
only the availability of sqrt was checked. This happens on odd platforms like
windows.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@189724 91177308-0d34-0410-b5e6-96231b3b80d8