[XLA:GPU] Add an implementation of scatter for GPU

This simple has a kernel that runs on every element of the updates tensor,
figure out the right indices to perform the update, and applies it with an
atomic operation.

Currently we emit a CAS for plain (i.e. non-add) updates, which is inefficient.
Also TuplePointsToAnalysis doesn't know that it should alias the operand and
output buffers of a scatter, which would avoid a copy.

PiperOrigin-RevId: 216412467

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