Do not report MA0148/MA0149 when the constant does not convert to the operand type

[meziantou]

Problem

MA0148/MA0149 rewrite value == 1L (with int value) into value is 1L, which fails with CS0266. The binary equality operator promotes the int operand to long, but the fixer copies the operand syntax into an is expression, dropping the implicit conversion. Constant patterns are matched against the operand's own type, so the long constant is not valid there.

The existing guard only excluded implicit user-defined conversions, not built-in numeric promotion.

Fix

When the comparison implicitly converts the operand, the analyzer and the fixer now check that the constant is implicitly convertible (without user-defined conversion) to the type of the operand as written in source (or its underlying type when nullable). If not, the comparison is not reported nor rewritten. This check replaces HasImplicitUserDefinedConversion, which it covers.

• Still reported/fixed: byteValue == 1 → byteValue is 1 (same for short, ushort, byte?)
• No longer reported: int vs 1L / 1.0 / 1m, int? vs 1L, float vs 0.1, byte vs 300, char vs 65
• Merged chains keep the valid terms only: value == 1 || value == 2L → value is 1 || value == 2L

The fixer shares the check for both single comparisons and merged ||/&& candidates, so a fix triggered on a valid term can no longer merge an invalid one.

Note for reviewers

The operand type is taken from SemanticModel.GetTypeInfo(operandSyntax).Type rather than by unwrapping the conversion operations: Roslyn 4.8 represents (DayOfWeek?)1 as a chain of implicit conversions down to the int literal, so unwrapping overshoots and broke the existing EqualityComparison_NullableEnum test on that version.

Tests

• New tests in UsePatternMatchingForEqualityComparisonsAnalyzerTests for non-convertible constants, convertible promoted operands, and mixed/merged comparisons (verified that 9 of the new cases fail without the fix).
• MA0148/MA0149 tests pass on Roslyn 4.8, 4.14, 5.0, 5.6 and 5.9 (46/46 each).
• Full test suite passes on Roslyn 5.9 (4394/4394).
• docs/Rules/MA0148.md and docs/Rules/MA0149.md document the new exclusion; DocumentationGenerator reports no further changes.