Emit rem.un for unsigned modulo expressions

[Copilot]

CompileFast() was emitting signed remainder IL for Expression.Modulo in unsigned scenarios, which breaks results when a uint operand has its high bit set. The regression showed up with block-local uint variables, where the standard compiler/interpreter still produced the correct unsigned remainder.

• Codegen

  • Update arithmetic opcode selection for ExpressionType.Modulo
  • Emit OpCodes.Rem_Un when the expression result type is an unsigned primitive
  • Preserve existing signed behavior for non-unsigned modulo

• Regression coverage

  • Add a focused arithmetic test for a block-local uint
  • Use 0x80000000u % 3u to exercise the signed/unsigned distinction
  • Assert both the runtime result and the generated IL opcode

• Scenario covered

  var b = Expression.Parameter(typeof(uint), "b");
  var a = Expression.Variable(typeof(uint), "a");
  
  var expr = Expression.Lambda<Func<uint, uint>>(
      Expression.Block(new[] { a },
          Expression.Assign(a, Expression.Constant(0x80000000u)),
          Expression.Modulo(a, b)),
      b);

  This path should produce 2u and emit rem.un, not rem.

[kevinferrare]

@dadhi Wow thanks for the quick fix!

Do you think it would be possible to do a release with the fix?

[dadhi]

Sure. Soon.