Fix ICE when emitting event with indexed function type requiring implicit conversion

[chfast]

The IR codegen asserted exact type equality between the argument and event parameter types for indexed function-type parameters (IRVariable(arg).type() == *functionType). This fails when the argument has a different but implicitly convertible state mutability (e.g. function() external pure passed to an event expecting function() external view indexed), since FunctionType::operator== compares mutability.

Replace the strict equality check with individual structural assertions matching the legacy codegen (ExpressionCompiler.cpp:999-1009): verify both types are external function types and the argument has no bound first argument. The underlying combineExternalFunctionIdFunction operates on raw (address, selector) stack values and is unaffected by mutability differences.

The legacy codegen is not affected — it already checks the argument and parameter types separately without requiring equality.

Fixes #16298

[github-actions]

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