[InstCombine] foldICmpBinOp(): consider inverted check in 'unsigned add overflow' check

A follow-up for r342004.
This will be changed to produce @llvm.add.with.overflow in a later patch,
but for now just make things more consistent overall.

https://rise4fun.com/Alive/qxE

Name: (Op1 + X) u< Op1 --> ~Op1 u< X
  %t0 = add i8 %Op1, %X
  %r = icmp ult i8 %t0, %Op1
=>
  %n = xor i8 %Op1, -1
  %r = icmp ult i8 %n, %X

Name: (Op1 + X) u>= Op1 --> ~Op1 u>= X
  %t0 = add i8 %Op1, %X
  %r = icmp uge i8 %t0, %Op1
=>
  %n = xor i8 %Op1, -1
  %r = icmp uge i8 %n, %X

;-------------------------------------------------------------------------------

Name: Op0 u> (Op0 + X) --> X u> ~Op0
  %t0 = add i8 %Op0, %X
  %r = icmp ugt i8 %Op0, %t0
=>
  %n = xor i8 %Op0, -1
  %r = icmp ugt i8 %X, %n

Name: Op0 u<= (Op0 + X) --> X u<= ~Op0
  %t0 = add i8 %Op0, %X
  %r = icmp ule i8 %Op0, %t0
=>
  %n = xor i8 %Op0, -1
  %r = icmp ule i8 %X, %n

llvm-svn: 371100