Let EarlyCSE fold equivalent freeze instructions

Summary:
This patch makes EarlyCSE fold equivalent freeze instructions.

Another optimization that I think will be useful is to remove freeze if its operand is used as a branch condition or at llvm.assume:

```
  %c = ...
  br i1 %c, label %A, ..
A:
  %d = freeze %c ; %d can be optimized to %c because %c cannot be poison or undef (or 'br %c' would be UB otherwise)
```

If it make sense for EarlyCSE to support this as well, I will make a patch for this.

Reviewers: spatel, reames, lebedev.ri

Reviewed By: lebedev.ri

Subscribers: lebedev.ri, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D75334