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.

There will be test cases for this added to the R600 backend in a
future commit.

llvm-svn: 214466