[ScheduleOptimizer] Hoist extension nodes after schedule optimization.


[ScheduleOptimizer] Hoist extension nodes after schedule optimization.

Extension nodes make schedule trees are less flexible: Many operations,
such as rescheduling, do not work on such schedule trees with extension.
As such, some functionality such as determining parallel loops in isl's
AST are disabled.

Currently, only the pattern-matching generalized matrix-matrix
multiplication optimization adds extension nodes (to add copy-in

This patch removes all extension nodes as the last step of the schedule
optimization by hoisting the extension node's added domain up to the
root domain node. All following passes can assume that schedule trees
work without restrictions, including the parallelism test. Mark the
outermost loop of the optimized matrix-matrix multiplication as parallel
such that -polly-parallel is able to parallelize that loop.

Differential Revision: https://reviews.llvm.org/D58202