swift234

Split the lookup node lookup cold path from the hot path.

In debug builds getGenericMetadata takes about 10% of the execution time.
Most of this time is spent in searching the binary tree. This commit splits
the search into cold path and hot path, where the less likely checks are
done after the mode likely checks. This reduces the overall runtime of the
test suite in -Onone by ~1%.

Swift SVN r23419