[AArch64] Add an AArch64 pass for loop idiom transformations (#72273)

We have added a new pass that looks for loops such as the following:

```
  while (i != max_len)
      if (a[i] != b[i])
          break;

  ... use index i ...
```

Although similar to a memcmp, this is slightly different because instead
of returning the difference between the values of the first non-matching
pair of bytes, it returns the index of the first mismatch. As such, we
are not able to lower this to a memcmp call.

The new pass can now spot such idioms and transform them into a
specialised predicated loop that gives a significant performance
improvement for AArch64. It is intended as a stop-gap solution until
this can be handled by the vectoriser, which doesn't currently deal with
early exits.

This specialised loop makes use of a generic intrinsic that counts the
trailing zero elements in a predicate vector. This was added in
https://reviews.llvm.org/D159283 and for SVE we end up with brkb & incp
instr...