ScheduleDAG: record PhysReg dependencies represented by CopyFromReg nodes

x86's CMPXCHG -> EFLAGS consumer wasn't being recorded as a real EFLAGS
dependency because it was represented by a pair of CopyFromReg(EFLAGS) ->
CopyToReg(EFLAGS) nodes. ScheduleDAG was expecting the source to be an
implicit-def on the instruction, where the result numbers in the DAG and the
Uses list in TableGen matched up precisely.

The Copy notation seems much more robust, so this patch extends ScheduleDAG
rather than refactoring x86.

Should fix PR20376.