Add test for compound assignment with swizzle

[sudonatalie]

Add a test for compound assignment to a vector with a swizzle on the right hand side. This case covers the bug reported in https://crbug.com/480933709 and will be fixed in WebGPU with https://dawn-review.googlesource.com/c/dawn/+/288435.

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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 2da1af1):

+webgpu:shader,validation,decl,assignment_statement:compound_assignment_with_swizzle:* - 1 cases, 1 subcases (~1/case)
-TOTAL: 285934 cases, 2344827 subcases
+TOTAL: 285935 cases, 2344828 subcases