Reduce the size of a DomTreeNodeBase from 80 to 56 bytes by not storing
the children in a SmallVector. Instead, store children as forward-linked
list. This also avoids extra allocations for nodes with many children.
Additionally, DomTreeNodeBase is now trivially destructible.
A lot of code depends on the order of nodes in the dominator tree, so
make sure that the order is the same when inserting nodes. (Not having
to do this would save 8 bytes per node.)
NewGVN uses the order of nodes in the dominator tree in a way that is
not entirely clear to me (https://reviews.llvm.org/D28129). I kept the
semantics as, but now this is the only external user of
addChild/removeChild, which actually should be private.
https://llvm-compile-time-tracker.com/compare.php?from=263802c56b4db3fc9b6ed9fd313499cb03ca44da&to=43e0c0c5b663b3a4067252fc0addbaccefd0014d&stat=instructions:u
284ef1b4ac