[GlobalISel][AArch64] Fold G_SUB into G_ICMP when it's safe to do so

Basically porting over the behaviour in AArch64ISelLowering to GISel. See
emitComparison for reference.

When we have something like this:

```
  lhs = G_SUB 0, y
  ...
  G_ICMP lhs, rhs
```

We can fold away the G_SUB and produce a cmn instead, given that we produce
the same value in NZCV.

Add a test showing that the transformation works, and also showing that we
don't perform the transformation when it's unsafe.

Also factor out the CSet emission into emitCSetForICMP.

Differential Revision: https://reviews.llvm.org/D63163

llvm-svn: 363596