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.

Fix CopyPropagation regression (2)

Remaining part of MIR copyprop regression by (I think) rust-lang#45380, which I missed in rust-lang#45753.

```rust
fn foo(mut x: i32) -> i32 {
    let y = x;
    x = 123; // `x` is assigned only once in MIR, but cannot be propagated to `y`
    y
}
```

So any assignment to an argument cannot be propagated.