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

   minnum(X, LesserC) == C --> false
   minnum(X, LesserC) >= C --> false
   minnum(X, LesserC) >  C --> false
   minnum(X, LesserC) != C --> true
   minnum(X, LesserC) <= C --> true
   minnum(X, LesserC) <  C --> true

maxnum siblings will follow if there are no problems here.

We should be able to perform some other combines when the constants
are equal or greater-than too, but that would go in instcombine.

We might also generalize this by creating an FP ConstantRange
(similar to what we do for integers).

Differential Revision: https://reviews.llvm.org/D61691

llvm-svn: 360899