Perform factorization as a last resort of unsafe fadd/fsub simplification.

Rules include:
  1)1 x*y +/- x*z => x*(y +/- z) 
    (the order of operands dosen't matter)

  2) y/x +/- z/x => (y +/- z)/x 

 The transformation is disabled if the new add/sub expr "y +/- z" is a 
denormal/naz/inifinity.

rdar://12911472


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@177088 91177308-0d34-0410-b5e6-96231b3b80d8