[InstCombine] simplify min/max of min/max with same operands (PR35607)

This is the original integer variant requested in:
https://bugs.llvm.org/show_bug.cgi?id=35607

As noted in the TODO and several similar TODOs around this block,
we could do this in instsimplify, but then it would cost more
because we would be trying to match min/max via ValueTracking
in 2 different places.

There are 4 commuted variants for each of smin/smax/umin/umax
that are not matched here. There are also icmp predicate variants
that are not included in the affected test file because they are
already handled by instsimplify by folding the final icmp to
true/false.

https://rise4fun.com/Alive/3KVc

  Name: smax(smax, smin)
  %c1 = icmp slt i32 %x, %y
  %c2 = icmp slt i32 %y, %x
  %min = select i1 %c1, i32 %x, i32 %y
  %max = select i1 %c2, i32 %x, i32 %y
  %c3 = icmp sgt i32 %max, %min
  %r = select i1 %c3, i32 %max, i32 %min
  =>
  %r = %max

  Name: smin(smax, smin)
  %c1 = icmp slt i32 %x, %y
  %c2 = icmp slt i32 %y, %x
  %min = select i1 %c1, i32 %x, i32 %y
  %max = select i1 %c2, i32 %x, i32 %y
  %c3 = icmp sgt i32 %max, %min
  %r = select i1 %c3, i32 %min, i32 %max
  =>
  %r = %min

  Name: umax(umax, umin)
  %c1 = icmp ult i32 %x, %y
  %c2 = icmp ult i32 %y, %x
  %min = select i1 %c1, i32 %x, i32 %y
  %max = select i1 %c2, i32 %x, i32 %y
  %c3 = icmp ult i32 %min, %max
  %r = select i1 %c3, i32 %max, i32 %min
  =>
  %r = %max

  Name: umin(umax, umin)
  %c1 = icmp ult i32 %x, %y
  %c2 = icmp ult i32 %y, %x
  %min = select i1 %c1, i32 %x, i32 %y
  %max = select i1 %c2, i32 %x, i32 %y
  %c3 = icmp ult i32 %min, %max
  %r = select i1 %c3, i32 %min, i32 %max
  =>
  %r = %min

llvm-svn: 369386