Auto merge of #45753 - sinkuu:mir_copyprop_arg, r=arielb1

Auto merge of #45753 - sinkuu:mir_copyprop_arg, r=arielb1

Fix MIR CopyPropagation errneously propagating assignments to function args

Compiling this code with MIR CopyPropagation activated will result in printing `5`,
because CopyProp errneously propagates the assignment of `5` to all `x`:

```rust
fn bar(mut x: u8) {
    println!("{}", x);
    x = 5;
}

fn main() {
    bar(123);
}

```

If a local is propagated, it will result in an ICE at trans due to an use-before-def:

```rust
fn dummy(x: u8) -> u8 { x }

fn foo(mut x: u8) {
    x = dummy(x); // this will assign a local to `x`
}
```
Currently CopyProp conservatively gives up if there are multiple assignments to a local,
but it is not took into account that arguments are already assigned from the beginning.
This PR fixes the problem by preventing propagation of assignments to function arguments.