[GVN] Fold equivalent freeze instructions

Summary:
This patch defines two freeze instructions to have the same value number if they are equivalent.

This is allowed because GVN replaces all uses of a duplicated instruction with another.

If it partially rewrites use, it is not allowed. e.g)

```
a = freeze(x)
b = freeze(x)
use(a)
use(a)
use(b)
=>
use(a)
use(b) // This is not allowed!
use(b)
```

Reviewers: fhahn, reames, spatel, efriedma

Reviewed By: fhahn

Subscribers: lebedev.ri, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D75398