[SDAG] Introduce a common MEMBARRIER node [nfc]

[SDAG] Introduce a common MEMBARRIER node [nfc]

We have multiple targets which have defined custom instructions and sdag nodes to represent a compiler memory barrier. This patch consolidates the sdag node definition into common code.

This is a companion to D92842, but a bit different in focus. This change consolidates the existing sdag node definitions; that patch skipped defining a sdag node by instead going straight to a target node. That patch is also not NFC - as being so is quite hard for commoning up the instruction definitions.

I started with two backends to ensure the new common code was reusable while not having a massive diff. Once this lands, I'll submit a series of NFCs for backends where the changes are obvious, or reviews if more discussion is needed.

Differential Revision: https://reviews.llvm.org/D141317