[InstCombine] Add support for maximum(a,b) + minimum(a,b) => a + b

Unfortunately alive2 cannot prove the correctness due to fails by timeout even for
float type half.

However it should be correct. If a and b are not NaN, maximum and minimum will just
return different values (a and b) and take into account a + b == b + a this is the same.
If a or b is NaN, than maximum and minimum are equal to NaN and NaN + NaN is NaN.
a + b is also a NaN.

In terms of preserving fast flags, we cannot preserve ninf due to
minimum(NaN, Infinity) == maximum(NaN, Infinity) == NaN,
minimum(NaN, Infinity) +ninf maximum(NaN, Infinity) == NaN +ninf NaN = NaN
However transformation will change
minimum(NaN, Infinity) + maximum(NaN, Infinity) to NaN +ninf Infinity == poison.

But if fadd is marked as nnan, we can preserve because NaN +ninf/nnan NaN = poison as well.

The same optimization for
  maximum(a,b) * minimum(a,b) => a * b
is added.
All said above for fadd is correct for fmul.

Reviewed By: mkazantsev
Differential Revision: https://reviews.llvm.org/D147299