workgroupUniformLoad: test pointer-to-atomic variant

[dneto0]

See gpuweb/gpuweb#5141

This requires basic testing:

• that all invocations get the same value from the atomic
• that it passes uniformity checks (the load is treated as uniform)

Ideally we would also modify the memory model tests to stress test the case. (That would likely be on me, as I'm the maintainer of those tests)

[dj2]

I think this is done in d3ac578 already?

[dneto0]

#4332
oh, I didn't now about that!

It's a good start.

• I'd also like to see loading from an atomic inside a larger workgroup variable. The Add tests for workgroupUniformLoad overload #4332 only tests the case where the atomic is the whole store type to the variable.
• extend uniformity tests (my second point above).

[dneto0]

@beaufortfrancois FYI in case you wanted to pursue this further

[beaufortfrancois]

@dneto0 I'll let you handle it if that's okay. If not, I can start working on it.

[beaufortfrancois]

@dneto0 I've started #4347 and #4348

[beaufortfrancois]

Let me know if we can close this issue or if we shall wait for @dneto0 to modify the memory model tests as well.

[dneto0]

Please keep this open for the memory model stress tests. Otherwise I'd open a new one for the stress tests... :-)