shader/validation: Add compound assignment validation

Extend the binary operator expression validation to also validate compound assignment.

Also calculate the binary operator's resulting type, and use that to validate the result type is as expected.

Issue: #1644

Requirements for PR author:

[x] All missing test coverage is tracked with "TODO" or .unimplemented().
[x] New helpers are /** documented */ and new helper files are found in helper_index.txt.
[x] Test behaves as expected in a WebGPU implementation. (If not passing, explain above.)
[x] Test have be tested with compatibility mode validation enabled and behave as expected. (If not passing, explain above.)

Requirements for reviewer sign-off:

[ ] Tests are properly located in the test tree.
[ ] Test descriptions allow a reader to "read only the test plans and evaluate coverage completeness", and accurately reflect the test code.
[ ] Tests provide complete coverage (including validation control cases). Missing coverage MUST be covered by TODOs.
[ ] Helpers and types promote readability and maintainability.

When landing this PR, be sure to make any necessary issue status updates.

gpuweb / cts