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

[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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@238186 91177308-0d34-0410-b5e6-96231b3b80d8