In makestab, there's no way to have an illegal/unknown typ value for the 901 case, since all of its possible values are already controlled inside of the library via prior calls to tabsub and tabent. I suspect the 902 case may also be unreachable, since any overflow of MAXJL would likely have been flagged earlier by the similar bort within inctab.
In tabent, there's no way to hit the 900 case, since this could only happen if a mnemonic was longer than 8 characters, but that would have already been flagged earlier by calling nemock from within rdusdx, seqsdx, sntbbe or sntbde. Similarly, the 901 case is also unreachable since tabent is only ever called internally by tabsub with a tab value of either 'B' or 'D'.
Part of #445
This PR also removes some unreachable bort tests: