Implement WGSL uniformity analysis

[jimblandy]

Naga should implement the uniformity analysis introduced in gpuweb/gpuweb@0561a0ee.

I think this would replace the uniformity analysis we have now. It could still be enabled by ValidationFlags::CONTROL_FLOW_UNIFORMITY.

The algorithm given in the spec looks, at first glance, like it should be straightforward to implement on Naga's IR, even though we don't have handles/indices for statements, only expressions.

There are also some more ideas on how to update our analysis in #4320.

Items to check:

• DISABLE_UNIFORMITY_REQ_FOR_FRAGMENT_STAGE should be removed.
• Uniformity should be required for workgroupUniformLoad.
• Non-uniformity in unreachable code (e.g. additional statements after return / break / discard / continue) should be permitted.

[ErichDonGubler]

Firefox downstream tracking issue: https://bugzilla.mozilla.org/show_bug.cgi?id=1850044