[GlobalISel] Combine (x op 0) -> x for operations with a right identity of 0

[GlobalISel] Combine (x op 0) -> x for operations with a right identity of 0

Implement identity combines for operations like the following:

```
%a = G_SUB %b, 0
```

This can just be replaced with %b.

Over CTMark, this gives some minor size improvements at -O3.

Differential Revision: https://reviews.llvm.org/D76640