ORC: prepare for another patent-pending optimization (#15996)

* ORC: prepare for another patent-pending optimization

* bugfix

* '=copy' for refs can take a cyclic parameter for more ORC optimizations

* ORC: exploit the common 'it = it.next' pattern

* can't hurt to check for nil

* use an algorithm that is not obviously broken

* restore the test case

* final cleanups for --gc:orc

lib/system/cyclebreaker.nim

@@ -70,10 +70,12 @@ template color(c): untyped = c.rc and colorMask
 template setColor(c, col) =
   c.rc = c.rc and not colorMask or col
 
-proc nimIncRefCyclic(p: pointer) {.compilerRtl, inl.} =
+proc nimIncRefCyclic(p: pointer; cyclic: bool) {.compilerRtl, inl.} =
   let h = head(p)
   inc h.rc, rcIncrement
 
+proc nimMarkCyclic(p: pointer) {.compilerRtl, inl.} = discard
+
 type
   GcEnv = object
     traceStack: CellSeq