Extend coverage for lowering wide vector types during type legalization to allow us to use PACKSS/PACKUS patterns instead of dropping down to shuffle lowering.
First step towards avoiding premature folds of TRUNCATE to PACKSS/PACKUS nodes as described on Issue #63710 - which causes a large number of regressions on D152928 - we will next need to tweak the TRUNCATE widening in ReplaceNodeResults
Why no v2i64 and v4i64?