More closely match improveShuffleKindFromMask's shuffle ordering by
trying to match a SK_InsertSubvector shuffles patterns before SK_Select
- both can match many of the same patterns, but its much easier to
recognise when a SK_InsertSubvector can be converted to SK_Select than
vice-versa.
Another step towards #145335 - which I'm hoping will allow us to
generalise improveShuffleKindFromMask and remove getInstructionCost's
shuffle matching entirely.
e25db2f6b3