Test swizzle assignment eval order

[sudonatalie]

Test that the lhs swizzle's vector pointer is evaluated before the rhs. For a regular assignment, the vector value can be loaded after rhs evaluation (because rhs side effects on the target components will be overwritten anyways). However for a compound assignment, the lhs value of the swizzle must be loaded before rhs evaluation, and the vector should be re-loaded after rhs evaluation for use as the "old components" in the store of the whole result vector, because side effects from the rhs on non-swizzled components should persist.

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Requirements for PR author:

• All missing test coverage is tracked with "TODO" or .unimplemented().
• New helpers are /** documented */ and new helper files are found in helper_index.txt.
• Test behaves as expected in a WebGPU implementation. (If not passing, explain above.)
• 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.
• Test descriptions are accurate and complete.
• Tests provide complete coverage (including validation control cases). Missing coverage MUST be covered by TODOs.
• Tests avoid over-parameterization (see case count report).

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

[github-actions]

Results for build job (at a72a5dd):

+webgpu:shader,execution,statement,swizzle_assignment:eval_order:* - 1 cases, 1 subcases (~1/case)
+webgpu:shader,execution,statement,swizzle_assignment:compound_eval_order:* - 1 cases, 1 subcases (~1/case)
-TOTAL: 280593 cases, 2321592 subcases
+TOTAL: 280595 cases, 2321594 subcases