[ValueTracking] recognize min/max-of-min/max with notted ops (PR35875)

This was originally planned as the fix for:
https://bugs.llvm.org/show_bug.cgi?id=35834
...but simpler transforms handled that case, so I implemented a 
lesser solution. It turns out we need to handle the case with 'not'
ops too because the real code example that we are trying to solve:
https://bugs.llvm.org/show_bug.cgi?id=35875
...has extra uses of the intermediate values, so we can't rely on 
smaller canonicalizations to get us to the goal.

As with rL321672, I've tried to show every possibility in the
codegen tests because that's the simplest way to prove we're doing
the right thing in the wide variety of permutations of this pattern.

We can also show an InstCombine win because we added a fold for
this case in:
rL321998 / D41603

An Alive proof for one variant of the pattern to show that the 
InstCombine and codegen results are correct:
https://rise4fun.com/Alive/vd1

Name: min3_nots
  %nx = xor i8 %x, -1
  %ny = xor i8 %y, -1
  %nz = xor i8 %z, -1
  %cmpxz = icmp slt i8 %nx, %nz
  %minxz = select i1 %cmpxz, i8 %nx, i8 %nz
  %cmpyz = icmp slt i8 %ny, %nz
  %minyz = select i1 %cmpyz, i8 %ny, i8 %nz
  %cmpyx = icmp slt i8 %y, %x
  %r = select i1 %cmpyx, i8 %minxz, i8 %minyz
=>
  %cmpxyz = icmp slt i8 %minxz, %ny
  %r = select i1 %cmpxyz, i8 %minxz, i8 %ny

Name: min3_nots_alt
  %nx = xor i8 %x, -1
  %ny = xor i8 %y, -1
  %nz = xor i8 %z, -1
  %cmpxz = icmp slt i8 %nx, %nz
  %minxz = select i1 %cmpxz, i8 %nx, i8 %nz
  %cmpyz = icmp slt i8 %ny, %nz
  %minyz = select i1 %cmpyz, i8 %ny, i8 %nz
  %cmpyx = icmp slt i8 %y, %x
  %r = select i1 %cmpyx, i8 %minxz, i8 %minyz
=>
  %xz = icmp sgt i8 %x, %z
  %maxxz = select i1 %xz, i8 %x, i8 %z
  %xyz = icmp sgt i8 %maxxz, %y
  %maxxyz = select i1 %xyz, i8 %maxxz, i8 %y
  %r = xor i8 %maxxyz, -1

llvm-svn: 322283