TableGen: Allow AddedComplexity values to be negative

TableGen: Allow AddedComplexity values to be negative

This is useful for cases when stand-alone patterns are preferred to the
patterns included in the instruction definitions.  Instead of requiring
that stand-alone patterns set a larger AddedComplexity value, which
can be confusing to new developers, the allows us to reduce the
complexity of the included patterns to achieve the same result.

llvm-svn: 213521