[release/8.0] Support assignment to multiple refs in trim analyzer

[github-actions]

Backport of #90287 to release/8.0

/cc @sbomer

Customer Impact

Analyzer would throw an exception, producing the following error with certain specific code patterns involving assignments to multiple refs:

'ILLink.RoslynAnalyzer.DynamicallyAccessedMembersAnalyzer' threw an exception of type 'System.InvalidOperationException' with message 'Operation is not valid due to the current state of the object.'.

For example, the following code would produce this error:

(b ? ref f1 : ref f2) = v;

Testing

Added testcases to validate that this and similar patterns no longer throw and produce reasonable analysis results (even though it is unlikely that such a pattern will be used in a way that is meaningful for the purpose of trim warnings).

Risk

Low risk. This fixes an analyzer crash without changing the analysis results for existing code. There's a fair amount of code in this change, but any risk is limited to the analyzer that runs during build, not directly affecting the runtime behavior or the output binary.

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Issue Details

---

Backport of #90287 to release/8.0

/cc @sbomer

Customer Impact

Testing

Risk

IMPORTANT: If this backport is for a servicing release, please verify that:

• The PR target branch is release/X.0-staging, not release/X.0.

• If the change touches code that ships in a NuGet package, you have added the necessary package authoring and gotten it explicitly reviewed.

Author:	github-actions[bot]
Assignees:	-
Labels:	linkable-framework, area-Tools-ILLink
Milestone:	-

[jeffschwMSFT]

approved. once you have a code review and a green ci you can merge.

[carlossanlop]

@sbomer I can merge it for you, just ping me. No need to click on "Update branch" (it will restart the CI).

There is a CI failure in browser-wasm saying there's not enough disk space, so I assume it's unrelated. I re-ran the failed legs just in case, but they were showing up as warnings so I might just merge if they happen again.