[SimplifyCFG] Check if the return instruction causes undefined behavior

[SimplifyCFG] Check if the return instruction causes undefined behavior

This should fix https://github.com/rust-lang/rust/issues/107681.

Return undefined to a noundef return value is undefined.

Example:

```
define noundef i32 @test_ret_noundef(i1 %cond) {
entry:
  br i1 %cond, label %bb1, label %bb2
bb1:
  br label %bb2
bb2:
  %r = phi i32 [ undef, %entry ], [ 1, %bb1 ]
  ret i32 %r
}
```

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D144319