Revert "Fixed issue with sign extension with I type"

This reverts commit c964418c02a55c73d81d713bdb161e2671fd6712 (PR #2).

The Altera docs clearly state that the ADDI instruction sign-extends the 16 bit immediate prior to addition. Thus, the original example is wrong.

This seems to fix tests: after reverting, all tests pass; before reverting, only 18 tests passed.