I suspect this is common across many companies; even if there was a "fire visa first" policy, the actual managers who decide who gets laid off would likely lean away from laying off someone on a visa, because of the issues it could cause for the person.

Now there can be a snide comment about how the visa workers are likely "better bang for the buck" for the company, and the company may have a hard time arguing they need more visas next year if they just laid non-visa workers off.