[AArch64][GlobalISel] Fold shifts into G_ICMP

Since G_ICMP can be selected to a SUBS, we can fold shifts into such compares.

E.g.

```
cmp	w1, w0, lsl #3
cmp	w1, w0, lsr #3
cmp	w1, w0, asr #3
```

This is done the same way as for adds and subtracts, using
`selectShiftedRegister`.

This gives some minor code size savings on CTMark.

https://reviews.llvm.org/D79365