Test index into swizzle for assignment

[sudonatalie]

Array-like indexing into swizzle views was added to the swizzle assignment WGSL language feature specification in a recent change to the spec PR: gpuweb/gpuweb@4b8daf8

These new cases have been tested against the updated dawn implementation at: https://dawn-review.googlesource.com/c/dawn/+/327436

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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 56bea06):

-webgpu:shader,execution,statement,swizzle_assignment:swizzle_assignment_vars:* - 16 cases, 128 subcases (~8/case)
-webgpu:shader,execution,statement,swizzle_assignment:swizzle_compound_assignment:* - 4 cases, 32 subcases (~8/case)
+webgpu:shader,execution,statement,swizzle_assignment:swizzle_assignment_vars:* - 21 cases, 168 subcases (~8/case)
+webgpu:shader,execution,statement,swizzle_assignment:swizzle_compound_assignment:* - 7 cases, 56 subcases (~8/case)
+webgpu:shader,execution,statement,swizzle_assignment:dynamic_index_into_swizzle:* - 30 cases, 228 subcases (~8/case)
+webgpu:shader,execution,statement,swizzle_assignment:indexed_swizzle_eval_order:* - 1 cases, 1 subcases (~1/case)
+webgpu:shader,execution,statement,swizzle_assignment:indexed_swizzle_compound_eval_order:* - 1 cases, 1 subcases (~1/case)
-webgpu:shader,validation,statement,swizzle_assignment:invalid_index_into_swizzle_view:* - 1 cases, 1 subcases (~1/case)
+webgpu:shader,validation,statement,swizzle_assignment:index_into_swizzle_view:* - 30 cases, 30 subcases (~1/case)
+webgpu:shader,validation,statement,swizzle_assignment:dynamic_index_into_swizzle_view:* - 6 cases, 6 subcases (~1/case)
-TOTAL: 285244 cases, 2346076 subcases
+TOTAL: 285319 cases, 2346405 subcases

[sudonatalie]

Also modified tests to allow duplicates in the nested swizzle view being indexed, since this is actually allowed by the spec.