[InstSimplify] fold fcmp (maxnum, X, C1), C2

This is the sibling transform for rL360899 (D61691):

  maxnum(X, GreaterC) == C --> false
  maxnum(X, GreaterC) <= C --> false
  maxnum(X, GreaterC) <  C --> false
  maxnum(X, GreaterC) >= C --> true
  maxnum(X, GreaterC) >  C --> true
  maxnum(X, GreaterC) != C --> true

llvm-svn: 361118