[MachineLICM] Relax overlay conservative PHI check (#67186)

Skip LICM if PHI belongs to the current loop, e.g. is in the
loop's header. This prevents LICM from bailing for CFGs like

L1:
  R = LoopInvariant // can be LICM'd
  BR L1
L2:
  PHI(R, ..)
  BR L2