AArch64: Add HInstruction scheduling support.

This commit adds a new `HInstructionScheduling` pass that performs
basic scheduling on the `HGraph`.

Currently, scheduling is performed at the block level, so no
`HInstruction` ever leaves its block in this pass.

The scheduling process iterates through blocks in the graph. For
blocks that we can and want to schedule:
1) Build a dependency graph for instructions. It includes data
   dependencies (inputs/uses), but also environment dependencies and
   side-effect dependencies.
2) Schedule the dependency graph. This is a topological sort of the
   dependency graph, using heuristics to decide what node to schedule
   first when there are multiple candidates. Currently the heuristics
   only consider instruction latencies and schedule first the
   instructions that are on the critical path.

Test: m test-art-host
Test: m test-art-target

Change-Id: Iec103177d4f059666d7c9626e5770531fbc5ccdc