DeadStoreElimination can create calls where return and arguments overlap

DeadStoreElimination wants to turn copy arguments into move arguments, but that transformation is not always valid. The pass does not account for this case:

_1 = f(copy _1);

but we also need a callee with codegen that clearly relies on the return place and argument not overlapping. This works, but uses custom_mir for both:

#![feature(custom_mir, core_intrinsics)]
#![allow(internal_features)]

use std::intrinsics::mir::*;

#[repr(C)]
struct Big {
    a: u64,
    b: u64,
    c: u64,
    d: u64,
    e: u64,
}

#[inline(never)]
#[custom_mir(dialect = "runtime")]
fn f(x: Big) -> Big {
    mir! {
        type RET = Big;
        {
            RET.a = 1;
            RET.b = x.a;
            RET.c = 0;
            RET.d = 0;
            RET.e = 0;
            Return()
        }
    }
}

#[inline(never)]
#[custom_mir(dialect = "runtime")]
fn caller(x: Big) -> Big {
    mir! {
        {
            Call(x = f(x), ReturnTo(bb1), UnwindUnreachable())
        }
        bb1 = {
            RET = x;
            Return()
        }
    }
}

fn main() {
    let x = Big { a: 42, b: 0, c: 0, d: 0, e: 0 };
    let y = caller(x);
    assert_eq!(y.b, 42);
}