RegisterCoalescer: Remove dubious dropping of implicit virtual register defs

Don't understand why this would either be OK or necessary, but doesn't
appear to happen in any tests. This was introduced way back in
76e66c3

https://reviews.llvm.org/D156265

Author: arsenm

llvm/lib/CodeGen/RegisterCoalescer.cpp

@@ -1396,9 +1396,8 @@ bool RegisterCoalescer::reMaterializeTrivialDef(const CoalescerPair &CP,
     MachineOperand &MO = CopyMI->getOperand(I);
     if (MO.isReg()) {
       assert(MO.isImplicit() && "No explicit operands after implicit operands.");
-      // Discard VReg implicit defs.
-      if (MO.getReg().isPhysical())
-        ImplicitOps.push_back(MO);
+      assert(MO.getReg().isPhysical() && "unexpected implicit virtual register def");
+      ImplicitOps.push_back(MO);
     }
   }