Add tests for comparisons and shifts. Also add enough integer
conversions to compile the shift test (llshift_e.c), and disable
some wrong rules for ldc and conversions.
Skip the long-long test set on other platforms, because they don't
have long long. Each platform would need to implement 8-byte
operations like `adi 8` in its code generator, and set long long to
8 bytes in its descr file.
The first test is for negation, addition, and subtraction. It also
requires comparison for equality.