Missed optimization: failure to remove dead edge

[kevina]

From discourse discussion: https://discourse.llvm.org/t/missed-optimization-due-to-overflow-check/88700

Godbolt link: https://godbolt.org/z/xqW3rcnKd.

Relevant IR code.

_ZN4FracC2Eii.exit.i:
  %0 = phi i32 [ 1, %if.else.i.i ], [ -1, %if.end10.i.i ]
  ...

lor.lhs.false.i:
  switch i32 %0, label %if.end.i [
    i32 1, label %_ZN7ReducedC2Eii.exit
    i32 -1, label %_ZN7ReducedC2Eii.exit
  ]

if.end.i:
  call void @Reduced::reduce_in_place()(ptr noundef nonnull align 4 dereferenceable(8) %retval)
  br label %_ZN7ReducedC2Eii.exit

_ZN7ReducedC2Eii.exit:
  %1 = load i64, ptr %retval, align 8
  ret i64 %1
}

From %0 = phi i32 [ 1, %if.else.i.i ], [ -1, %if.end10.i.i ] it should be obvious that %0 can only be 1 or -1 yet LLVM doesn't see it and fails to remove the dead edge.