[X86] ReplaceNodeResults - truncate sub-128-bit vectors as shuffles directly (#83120)

We were scalarizing these truncations, but in most cases we can widen the source vector to 128-bits and perform the truncation as a shuffle directly (which will usually lower as a PACK or PSHUFB).

For the cases where the widening and shuffle isn't legal we can leave it to generic legalization to scalarize for us.

Fixes #81883