[TableGen] Fix line wrapping logic for the autogenerated header to use math...

[TableGen] Fix line wrapping logic for the autogenerated header to use math that makes more sense (at least to me).

The old code had a bug if the description was between 75 and 85 characters or so as it substracted PSLen from Desc.size() instead of MAX_LINE_LEN in the compare. It also calculated odd values for PosE on the last split and just let StringRef::slice take care of it being larger than the description string.

llvm-svn: 238187