[AArch64] Reorder Comparison Trees to Facilitate CSE (#168064)

The AArch64 backend converts trees formed by conjunctions/disjunctions
of comparisons into sequences of `CCMP` instructions. The implementation
before this change checks whether a sub-tree must be processed first. If
not, it processes the operations in the order they occur in the DAG.

This may not be optimal if there is a corresponding `SUB` node for one
of the comparisons. In this case, we should process this comparison
first because we can then use the same instruction for the `SUB` node
and the comparison.

To achieve this, this commit comprises the following changes:

- Extend `canEmitConjunction` with a new output parameter `PreferFirst`,
  which reports to the caller whether the sub-tree should preferably be
  processed first.
- Set `PreferFirst` to `true` if we can find a corresponding `SUB` node
  in the DAG.
- If we can process a sub-tree with `PreferFirst = true` first (i.e., we
  do not violate any `MustBeFirst` constraint by doing so), we swap the
  sub-trees.
- The already existing code for performing the common subexpression
  elimination takes care to use only a single instruction for the
  comparison and the `SUB` node if possible.

Closes #149685.