[backport] fix #15595 procvar == works in VM (#15724)

compiler/vm.nim

@@ -994,8 +994,10 @@ proc rawExecute(c: PCtx, start: int, tos: PStackFrame): TFullReg =
         let nb = regs[rb].node
         let nc = regs[rc].node
         if nb.kind != nc.kind: discard
-        elif (nb == nc) or (nb.kind == nkNilLit): ret = true
-        elif nb.kind == nkIntLit and nb.intVal == nc.intVal: # TODO: nkPtrLit
+        elif (nb == nc) or (nb.kind == nkNilLit): ret = true # intentional
+        elif sameConstant(nb, nc): ret = true
+          # this also takes care of procvar's, represented as nkTupleConstr, eg (nil, nil)
+        elif nb.kind == nkIntLit and nc.kind == nkIntLit and nb.intVal == nc.intVal: # TODO: nkPtrLit
           let tb = nb.getTyp
           let tc = nc.getTyp
           ret = tb.kind in PtrLikeKinds and tc.kind == tb.kind

tests/vm/tvmmisc.nim

@@ -205,6 +205,30 @@ block: # bug #13081
   static:
     doAssert j1.x1 == 12
 
+block: # bug #15595
+  proc fn0()=echo 0
+  proc fn1()=discard
+  proc main=
+    var local = 0
+    proc fn2()=echo local
+    var a0 = fn0
+    var a1 = fn1
+    var a2 = fn2
+    var a3: proc()
+    var a4: proc()
+    doAssert a0 == fn0 # bugfix
+    doAssert a1 == fn1 # ditto
+    doAssert a2 == fn2 # ditto
+
+    doAssert fn0 != fn1
+
+    doAssert a2 != nil
+    doAssert a3 == nil # bugfix
+
+    doAssert a3 == a4 # bugfix
+  static: main()
+  main()
+
 # bug #15363
 import sequtils