[PATCH] Remove ISD::MEMBARRIER node?

[Tim Northover]

Hi all,

I've been looking at atomic operations recently, and noticed that
ISD::MEMBARRIER seems to be a bit of an orphan. As far as I can see
nothing can generate it, but multiple targets nevertheless contain
code to handle it specifically.

It was a bit before my time, but some archaeology suggests it was the
primary barrier instruction from the bad old days when we thought that
a fence was all you needed for synchronisation.

The attached patch removes this node (no test changes were necessary
to pass!). Is there any reason I shouldn't commit it?

(N.b. I've left the ShouldFoldAtomicFences stuff in even though it's
unused after this patch because I intend to work on a set of valid
ATOMIC_FENCE combines this weekend).

Tim.
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