Report MA0192 when the compared flag is not a constant

[meziantou]

Fixes #1313

What changed

MA0192 (Use HasFlag instead of bitwise checks) only recognized bitwise flag checks whose flag was a constant enum member. (value & comparand) == comparand was not reported when comparand was a parameter, a local variable, or a field.

• New shared helper src/Meziantou.Analyzer/Rules/UseHasFlagMethodCommon.cs: AreEquivalentOperands structurally compares two operations and accepts only side-effect free references — parameters, locals, non-volatile fields (with an equivalent instance), and this/base. It is picked up by the code fixers project through the existing Rules/*Common.cs glob, so the analyzer and the fixer stay in sync.
• UseHasFlagMethodAnalyzer.TryGetEnumFlagReference and UseHasFlagMethodFixer.TryGetEnumFlagReference now return IOperation instead of IFieldReferenceOperation and fall back to that equivalence check when the flag is not a constant enum member.
• Constant-zero operands are excluded from the new path so MA0192 does not overlap with MA0201 (Do not use zero-valued enum flags in flag checks).

The issue repro is now reported and fixed to value.HasFlag(comparand), as are the reversed-operand ((comparand & value) == comparand) and != variants.

Why the restriction to side-effect free references

The code fix replaces two evaluations of the flag expression by a single one. Properties, method calls, and volatile fields may yield a different value on each evaluation, so they are not reported. Tests cover each of those cases.

Note for reviewers

The existing test HasFlagsExtensionZeroFlag_NoDiagnostic contained public static bool HasFlags(this MyEnum value, MyEnum flags) => (value & flags) == flags;, which is exactly the pattern this change is meant to report, so that line now expects MA0192. The subject of that test (no MA0201 on value.HasFlags(MyEnum.None)) is unchanged.

Verification

• Full test suite on the default Roslyn version: 3736 passed, 0 failed.
• The 33 UseHasFlagMethodAnalyzerTests pass on roslyn4.8, 4.14, 5.0, 5.6 and 5.9.
• dotnet run --project src/DocumentationGenerator exits 0 with no further markdown change after updating docs/Rules/MA0192.md.