Fix #3826: wrap an overflowing constant subexpression in unchecked()

[sailro]

Fixes #3826.

A common hand-written GetHashCode uses a local accumulator:

var hashCode = 1688038063;
hashCode = hashCode * -1521134295 + Type.GetHashCode();
...

Because hashCode is a local, every * -1521134295 is a runtime operation and
compiles fine. The decompiler inlines the accumulator into one nested expression,
so the leading 1688038063 * -1521134295 becomes a compile-time constant
subexpression. In C#, constant subexpressions are always evaluated in a checked
context regardless of the project's overflow setting, so the decompiled output
failed to compile with CS0220 ("operation overflows at compile time in checked
mode").

Fix

HandleBinaryNumeric now detects an unchecked binary operation whose result is a
compile-time constant that overflows in a checked context, and annotates it with
ExplicitUncheckedAnnotation so AddCheckedBlocks emits an explicit
unchecked(...) wrapper. This mirrors the existing handling of an overflowing
constant cast to nint/nuint (TranslatedExpression.cs).

Before:

return (1688038063 * -1521134295 + a) * -1521134295 + b;   // CS0220

After:

return (unchecked(1688038063 * -1521134295) + a) * -1521134295 + b;

Only the overflowing constant subexpression is wrapped; non-constant
multiplications are left as-is.

Test

Correctness/UncheckedConstants exercises the inlined-accumulator hash. Without
the fix the decompiled output fails to recompile (CS0220); with it the output
recompiles and produces identical results.

Real-world occurrence: DiffPlex DiffPiece.GetHashCode (net45).

[Copilot]

Pull request overview

Fixes decompiled C# output failing to recompile when an inlined “hash accumulator” turns into a compile-time constant subexpression that overflows in checked constant-evaluation (CS0220). The change ensures the decompiler emits an explicit unchecked(...) wrapper only for the overflowing constant subexpression, preserving original unchecked runtime semantics and keeping non-constant arithmetic unchanged.

Changes:

• Update ExpressionBuilder.HandleBinaryNumeric to apply AddCheckedBlocks.ExplicitUncheckedAnnotation for unchecked binary ops that become overflowing compile-time constants under checked evaluation.
• Add ConstantBinaryOperatorOverflows(...) helper to detect checked-context overflow for already-constant binary operations.
• Add a new correctness test case (UncheckedConstants) and wire it into CorrectnessTestRunner.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
ICSharpCode.Decompiler/CSharp/ExpressionBuilder.cs	Detects overflowing constant subexpressions in unchecked binary ops and forces explicit unchecked(...) emission via ExplicitUncheckedAnnotation.
ICSharpCode.Decompiler.Tests/TestCases/Correctness/UncheckedConstants.cs	Adds a repro-style hash-chain test that becomes a constant overflow after inlining.
ICSharpCode.Decompiler.Tests/CorrectnessTestRunner.cs	Registers the new UncheckedConstants correctness test in the test runner.

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