Added IRBuilder::CreateFPExtOrFPTrunc.

This method serves an analogous purpose to CreateZExtOrTrunc/CreateSExtOrTrunc
but for floating point types.

In detail, it provides a manner when one is handling conversions of floating
point types of automatically selecting fpext, fptrunc, or identity depending on
the relative bitsize of the source and destination types.

llvm-svn: 172957