Object downconversion in VM should not copy (#10378)

Hopefully the type-check phase already rejected all the invalid
conversions by the time we execute the VM bytecode.

Problem reported by chrisheller on the Nim Forum

compiler/vm.nim

@@ -399,6 +399,11 @@ proc opConv(c: PCtx; dest: var TFullReg, src: TFullReg, desttyp, srctyp: PType):
         dest.floatVal = toBiggestFloat(src.intVal)
       else:
         dest.floatVal = src.floatVal
+    of tyObject:
+      if srctyp.skipTypes(abstractRange).kind != tyObject:
+        internalError(c.config, "invalid object-to-object conversion")
+      # A object-to-object conversion is essentially a no-op
+      moveConst(dest, src)
     else:
       asgnComplex(dest, src)