Optimize SelectionDAG's AssignTopologicalOrder even further.

Completely eliminate the TopOrder std::vector. Instead, sort
the AllNodes list in place. This also eliminates the need to
call AllNodes.size(), a linear-time operation, before
performing the sort.

Also, eliminate the Sources temporary std::vector, since it
essentially duplicates the sorted result as it is being
built.

This also changes the direction of the topological sort
from bottom-up to top-down. The AllNodes list starts out in
roughly top-down order, so this reduces the amount of
reordering needed. Top-down is also more convenient for
Legalize, and ISel needed only minor adjustments.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@56867 91177308-0d34-0410-b5e6-96231b3b80d8

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