Implement the unsigned-source overflowing conversions in the interpreter

[kg]

• Implements the _un versions of conv_ovf.
• Fixes some incorrect/missing conversions in the original conv_ovfs I added in a previous PR.
• Expands the conv_ovf helper so it can do same-size and growing conversions with sign changes.

[kg]

Noticed some other problems with our conversions so I'm going to clean them up.

[Copilot]

Pull Request Overview

This PR adds support for unsigned-source overflow-checking conversions in the interpreter backend and corresponding compiler support, and adds a new unit test to validate these conversions.

• Introduces new conv.u8.u4 and various conv.ovf.* opcodes in intops.def
• Extends ConvOvfHelper and opcode dispatch in interpexec.cpp to handle unsigned-source cases
• Updates compiler.cpp to emit the new overflow opcodes for unsigned conversions
• Adds TestConvOvfUn (and console logging) in Interpreter.cs to cover unsigned overflow conversions

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
src/tests/JIT/interpreter/Interpreter.cs	Added TestConvOvfUn method and Console.WriteLine calls for clarity
src/coreclr/vm/interpexec.cpp	Enhanced ConvOvfHelper to support growing conversions with sign changes and added unsigned conversion cases
src/coreclr/interpreter/intops.def	Defined new unsigned-source overflow-checking opcodes
src/coreclr/interpreter/compiler.cpp	Updated code generator to emit the new unsigned conversion opcodes

Comments suppressed due to low confidence (2)

src/coreclr/interpreter/compiler.cpp:4078

• In the CEE_CONV_OVF_U_UN case on 32-bit targets, using INTOP_CONV_OVF_U4_I8 treats the source as signed. To properly implement 'un' (treat source as unsigned), replace this with INTOP_CONV_OVF_U4_U8.

#else

src/tests/JIT/interpreter/Interpreter.cs:1570

• [nitpick] Variables a, b, and c are non-descriptive; consider renaming them to clarify their purpose (e.g., resultFromU2, resultFromU4, resultFromU8).

            byte a = (byte)u2,

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[janvorli]

@kg can you please resolve the conflict so that we can merge the change?

[janvorli]

LGTM, thank you!

[kg]

I see "Merging is blocked: All comments must be resolved" but there are no unresolved comments on this PR. Does anyone know what causes that?

EDIT: Copilot had marked some unresolved comments as "outdated", making them invisible.