GVN: Avoid merging borrows from the dereferenced value

[dianqk]

The PR is separated from #147886.

When replacing _3 with _2, we have to know _2 is valid at _3 = *_1;.

let mut _2: &A;
let mut _3: &A;

_2 = *_1;
// ...
_3 = *_1;

r? cjgillot

[rustbot]

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

[dianqk]

@rustbot author

[dianqk]

Closing in favor of #150485.