[X86] Remove combineVectorTruncation and delay general vector trunc to lowering

Stop folding vector truncations to PACKSS/PACKUS patterns prematurely - another step towards Issue #63710. We still prematurely fold to PACKSS/PACKUS if there are sufficient signbits, that will be addressed in a later patch when we remove combineVectorSignBitsTruncation.

This required ReplaceNodeResults to extend handling of sub-128-bit results to SSSE3 (or later) cases, which has allowed us to improve vXi32->vXi16 truncations to use PSHUFB.

I also tweaked LowerTruncateVecPack to recognise widened truncation source operands so the upper elements remain UNDEF (otherwise truncateVectorWithPACK* will constant fold them to allzeros/allones values).