Revert "[LV] Remove `LoopVectorizationCostModel::useEmulatedMaskMemRefHack()`"

This reverts commit 77a0da92 as we've
received multiple reports of this significantly impacting performance,
in ways that don't seem to just be target specific cost models going
wrong. I would offer some reproducers, but the test changes here seem to
be full of them!

Reverting for now and hopefully we can remove the "hack" more carefully
as we go.