* [RELAY] Add MergeCompilerRegions pass

This pass is part of the flow to support creating compiler
regions with multiple outputs. It should be called after
AnnotateTarget and will merge together regions that share
the same target to create larger compiler regions that can
be off-loaded to external codegens.

This pass implements an algorithm to ensure that during the
merging, no data dependency issues are created. See the tests
for an example of this case.

Co-authored-by: Ramana Radhakrishnan  <ramana.radhakrishnan@arm.com>
Co-authored-by: Manupa Karunaratne    <manupa.karunaratne@arm.com>

Change-Id: Ibd99083564608d888482f57c5080109f3eefec88

* [RELAY] Annotate compiler_ends on each edge

This alters the behaviour of the AnnotateTarget
pass to enforce the property that all compiler
annotations exist along a single data flow edge.
Specifically, this means they should have exactly
one parent and one child.

Change-Id: I0e74803a77767f4f377d17755a13a74a30909797

* Fix comment

* Rebase *Node::make

* Moved block outside for loop

* Code style

* Update make API

* Remove comment

* Remove redundant 'else's

* Make one line

* Fix comment

* RefWrite

* Fix merge ordering

* Add the RFC example as a test

* [FIX] Fixed merging behaviour in AnnotateRegionSet

Deleting items from a list while iterating it seems to
result in undefined behaviour which sometimes segfaults.
This makes sure all the item deletion happens separately.

* Added checks

* Move comment

* Update comments