[InstCombine] reduce more unsigned saturated add with 'not' op

[InstCombine] reduce more unsigned saturated add with 'not' op

We want to use the sum in the icmp to allow matching with
m_UAddWithOverflow and eliminate the 'not'. This is discussed
in D51929 and is another step towards solving PR14613:
https://bugs.llvm.org/show_bug.cgi?id=14613

  Name: not op
  %notx = xor i32 %x, -1
  %a = add i32 %x, %y
  %c = icmp ult i32 %notx, %y
  %r = select i1 %c, i32 -1, i32 %a
  =>
  %a = add i32 %x, %y
  %c2 = icmp ult i32 %a, %y
  %r = select i1 %c2, i32 -1, i32 %a

  Name: not op ugt
  %notx = xor i32 %x, -1
  %a = add i32 %x, %y
  %c = icmp ugt i32 %y, %notx
  %r = select i1 %c, i32 -1, i32 %a
  =>
  %a = add i32 %x, %y
  %c2 = icmp ult i32 %a, %y
  %r = select i1 %c2, i32 -1, i32 %a

https://rise4fun.com/Alive/niom

(The matching here is still incomplete.)

llvm-svn: 354224