Query Optimizer: Add OUTER --> INNER join conversion

[alamb]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

LEFT OUTER, RIGHT OUTER, and FULL OUTER JOINs are often more expensive to evaluate and preclude other optimizations (such as pushing down predicates as can be seen in #1618)

As such, sophisticated optimizers will actually rewrite OUTER joins to INNER joins depending on the predicates of the query to improve performance

Describe the solution you'd like

Add an OptimzierPass pass that will attempt to convert OUTER joins to inner joins.

This will require some non trivial research to figure out under what conditions the joins can be rewritten / converted

Additional context
Relevant discussion: #1618 (comment)

You can see a version of this code in Spark here: https://github.com/apache/spark/blob/aaf0e5e71509a2324e110e45366b753c7926c64b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/joins.scala#L119-L135

[alamb]

Whoops -- this is a dupe of #1585