SPARK-2519 part 2. Remove pattern matching on Tuple2 in critical section...

...s of CoGroupedRDD and PairRDDFunctions

This also removes an unnecessary tuple creation in cogroup.
There are all kinds of stuff that's really hard to profile at runtime. For example, the JVM might decide not to JIT the code because it is longer. It might decide not to do it because of exceptions. Branch prediction might stop working because it has too many branches. Etc.

Again, I'm only in favor of this change because it is small, and it might have impact. If it is a big ugly change, I would be against it without profiling.