[MachineCombiner] Don't ignore PHI depths (#82025)

The depths of the Root and the NewRoot are to be compared in
MachineCombiner::improvesCriticalPathLen(), and while the call to
BlockTrace.getInstrCycles(*Root) includes the Depth of a PHI, for some
reason PHI nodes have been ignored in getOperandDef(). 

This patch removes the special handling of PHIs in getOperandDef() so that
Root and NewRoot get a fair comparison. This does not affect loop headers
as MachineTraceMetrics handles that case by ignoring incoming PHI edges.