x86_64: delete usages of avx2 `vpack?s??`

This instruction actually has fairly useless semantics, and even the
cases that were semantically correct could save 1 cycle of latency by
using a different sequnce involving the avx version instead.

Closes #25174
e313b387a0Jacob Young committed on 9/11/2025, 2:40:26 AMparent1a0a9d7
2 files changedLine totals unavailable