Due to the way fixed length SVE lowering works, we sometimes introduce
ext/trunc nodes very late, these nodes then immediately get converted
into target specific nodes (UUNPKLO/UZP1) before they get a chance to be
folded into a load/store.
This patch introduces target specific dag combines for these nodes so that
we can still create extending loads/truncating stores out of them.
Is this universally true across all predicate patterns? Given my comment below I'm wondering if you'll eventually just create fresh PTRUEs rather than have to answer this question.