[IRBuilder] add creators for FP with FMF; NFCI

Also, add a helper for the constant folder to reduce duplication.

It seems out-of-place for and/or to be doing simplifications here?
Otherwise, I could have used the helper on those opcodes too. 


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@325808 91177308-0d34-0410-b5e6-96231b3b80d8