[X86] ReplaceNodeResults - widen sub-128-bit vector truncations if it would...

[X86] ReplaceNodeResults - widen sub-128-bit vector truncations if it would allow them to use PACKSS/PACKUS

We currently just scalarize sub-128-bit vector truncations, but if the input vector has sufficient signbits/zerobits then we should try to use PACKSS/PACKUS with a widened vector with don't care upper elements. Shuffle lowering will struggle to detect this if we wait until the scalarization has been revectorized as a shuffle.

Another step towards issue #63710