[X86] combineBitcastvxi1 - don't prematurely create PACKSS nodes.

Similar to Issue #63710 - by truncating the v8i16 result with a PACKSS node before type legalization, we fail to make use of various folds that rely on TRUNCATE nodes.

This required tweaks to LowerTruncateVecPackWithSignBits to recognise when the truncation source has been widened and to more closely match combineVectorSignBitsTruncation wrt truncating with PACKSS/PACKUS on AVX512 targets.

One of the last stages before we can finally get rid of combineVectorSignBitsTruncation.