[InstCombine] allow fdiv with constant dividend folds with less than full -ffast-math

It's possible that we could allow this either 'arcp' or 'reassoc' alone, but this
should be conservatively better than what we have right now. GCC allows this with
only -freciprocal-math.

The last test is changed to show a case that is expected to fold, but we need D43398.

llvm-svn: 325533