Fix assertions generated by optCreateJumpTableImpliedAssertions

[EgorBo]

Fixes #122254

bug repro:

switch (X)
{
    case 2:
        // ...
        break;
    case 3:
        // ...
        break;
    case 4:
        // ...
        break;
    default:
        // "X >= 5" assertion was created here (which is incorrect, X can be 0 or 1 as well)  
        break;
}

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[Copilot]

Pull request overview

This PR fixes a bug in the JIT compiler's assertion propagation optimization where incorrect assertions were being generated for switch statement default cases when the switch expression involved an offset.

Changes:

• Fixed assertion generation logic in optCreateJumpTableImpliedAssertions to only create range assertions for default cases when there's no offset
• Added a regression test to verify the fix

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/coreclr/jit/assertionprop.cpp	Added condition to check offset == 0 before creating assertions for default case, with explanatory comment
src/tests/JIT/Regression/JitBlue/Runtime_122254/Runtime_122254.cs	Added regression test with switch statement to verify correct behavior when block value is 0