[TableGen] Rewrite an assert to not do a bunch unsigned math and then try to...

[TableGen] Rewrite an assert to not do a bunch unsigned math and then try to ensure the result is a positive number.

I think the fact that it was explicitly excluding 0 kept this from being a tautology. The exclusion of 0 for the old math was also a bug that's easily hit if the description gets split into multiple lines.

llvm-svn: 238186