Removing DropAndReplace MIR allowed more code to compile

[cjgillot]

Before #107844, assignments of Drop types were modeled using replaced the DropAndReplace MIR terminator. That PR changed the behaviour to use a plain Drop followed by a plain Assign.

This made more code compile, for instance (#107844 (comment)):

struct Foo<'a> {
    v: &'a mut (),
}

impl Drop for Foo<'_> {
    fn drop(&mut self) {}
}

fn bar() {
    let mut v = ();
    let mut x = Foo { v: &mut v };
    drop(x);
    x = Foo { v: &mut v };
}

Steps:

• List the changes in behaviour;
• Propose to t-lang to ratify them.

[jyn514]

note that #107844 is on stable as of last week