[Hexagon] Fix MO_JumpTable const extender conversion
Previously this case fell through to unreachable, so it is clearly not
covered by any test case in LLVM. It may be dynamically unreachable, in
fact. However, if it were to run, this is what it would logically do.
The assert suggests that the intended behavior was not to allow folding
offsets from jump table indices, which makes sense.