[X86] LowerTRUNCATE - improve handling during type legalization to PACKSS/PACKUS patterns

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

Differential Revision: https://reviews.llvm.org/D154592