constraint elimination missing a case related to != and ==

[regehr]

consider this code:

extern void side_effect(void);
void f(int v0, int v1, int v2) {
  if (v2 != v0) return;
  if (v0 < v1) return;
  if (v1 > v2) side_effect();
}

we compile this to:

define void @f(i32 noundef %v0, i32 noundef %v1, i32 noundef %v2)  {
entry:
  %cmp.not = icmp eq i32 %v2, %v0
  %cmp1 = icmp sge i32 %v0, %v1
  %or.cond.not11 = and i1 %cmp1, %cmp.not
  %cmp4 = icmp sgt i32 %v1, %v2
  %or.cond10 = and i1 %cmp4, %or.cond.not11
  br i1 %or.cond10, label %if.then5, label %if.end6

if.then5:                                         ; preds = %entry
  tail call void @side_effect() #2
  br label %if.end6

if.end6:                                          ; preds = %entry, %if.then5
  ret void
}

but that last conditional is always false: https://alive2.llvm.org/ce/z/LaLygS

this issue seems to involve both icmp eq and icmp ne:
https://gcc.godbolt.org/z/MK4xhMna6
https://alive2.llvm.org/ce/z/IXTadx

cc @fhahn @dtcxzyw

[fhahn]

I think we only add eq/ne facts to the unsigned system.

[zjaffal]

Taking a look

[fhahn]

Looks like there's a proposed draft patch #121423 by @zsrkmyn