Provide a place for targets to insert ILP optimization passes.

Move the early if-conversion pass into this group.

ILP optimizations usually need to find the right balance between
register pressure and ILP using the MachineTraceMetrics analysis to
identify critical paths and estimate other costs. Such passes should run
together so they can share dominator tree and loop info analyses.

Besides if-conversion, future passes to run here here could include
expression height reduction and ARM's MLxExpansion pass.