[GlobalISel] Combine G_SELECTs of the form (cond ? x : x) into x

When we find something like this:

```
%a:_(s32) = G_SOMETHING ...
...
%select:_(s32) = G_SELECT %cond(s1), %a, %a
```

We can remove the select and just replace it entirely with `%a` because it's
always going to result in `%a`.

Same if we have

```
%select:_(s32) = G_SELECT %cond(s1), %a, %b
```

where we can deduce that `%a == %b`.

This implements the following cases:

- `%select:_(s32) = G_SELECT %cond(s1), %a, %a` -> `%a`

- `%select:_(s32) = G_SELECT %cond(s1), %a, %some_copy_from_a` -> `%a`

- `%select:_(s32) = G_SELECT %cond(s1), %a, %b` -> `%a` when `%a` and `%b`
   are defined by identical instructions

This gives a few minor code size improvements on CTMark at -O3 for AArch64.

Differential Revision: https://reviews.llvm.org/D76523