[GlobalISel] Add commutative matchers for compares.

This adds:

* `m_c_GICmp`
* `m_c_GFCmp`

These work the same way as the standard matchers, but will also try to commute
the LHS and RHS of a compare to get a match.

E.g.

```
m_c_GICmp(m_Pred(...), m_GAdd(...), m_GSub(...))
```

Can match either of

```
icmp cc (add x, y), (sub a, b)
icmp swapped_cc (sub a, b), (add x, y)
```

Differential Revision: https://reviews.llvm.org/D135415