[InstSimplify] Extend handlng of fp min/max.

Add support the cases like
  m(m(X,Y),m'(X,Y)) => m(X,Y)
where m is one of maxnum, minnum, maximum, minimum and
m' is m or inverse of m.

alive2 correctness check:
maxnum(maxnum,maxnum) https://alive2.llvm.org/ce/z/kSyAzo
maxnum(maxnum,minnum) https://alive2.llvm.org/ce/z/Vra8j2
minnum(minnum,minnum) https://alive2.llvm.org/ce/z/B6h-hW
minnum(minnum,maxnum) https://alive2.llvm.org/ce/z/rG2u_b
maximum(maximum,maximum) https://alive2.llvm.org/ce/z/N2nevY
maximum(maximum,minimum) https://alive2.llvm.org/ce/z/23RFcP
minimum(minimum,minimum) https://alive2.llvm.org/ce/z/spHZ-U
minimum(minimum,maximum) https://alive2.llvm.org/ce/z/Aa-VE8

Reviewed By: dantrushin, RKSimon
Differential Revision: https://reviews.llvm.org/D147137