Add NOOP expr operator

During the tiling process we sometimes add tiles for the labels in
conditional expressions, that have no operator associated. However,
the operator in memory would then be 0, which is a valid operator type.

That didn't matter until I changed the order of LOAD and COPY, since COPY
is special-cased in the register allocator to make a reference - the
register allocator would then create a cycle in the union-find data
structure and loop forever. Oops.

One (valid) way to deal with it is to have pseudotiles initialized to an
invalid value like -1. But I think I tried that once and it ran into
problems (maybe due to invalid operator info).  So instead, I'm
declaring the 'zero' operator to be illegal, and also, I'm starting
nodes from 1, so that no valid reference is ever zero.

src/jit/expr.c

@@ -685,6 +685,9 @@ MVMJitExprTree * MVM_jit_expr_tree_build(MVMThreadContext *tc, MVMJitGraph *jg,
     MVM_VECTOR_INIT(tree->info,  256);
     MVM_VECTOR_INIT(tree->roots, 16);
 
+    /* ensure that all references are nonzero */
+    MVM_VECTOR_PUSH(tree->nodes, MVM_JIT_NOOP);
+
     tree->graph      = jg;
     tree->num_labels = 0;
     /* Hold indices to the node that last computed a value belonging
@@ -937,7 +940,7 @@ MVMJitExprTree * MVM_jit_expr_tree_build(MVMThreadContext *tc, MVMJitGraph *jg,
     }
 
  done:
-    if (tree->nodes_num > 0) {
+    if (tree->roots_num > 0) {
         active_values_flush(tc, tree, values, sg->num_locals);
         MVM_jit_expr_tree_analyze(tc, tree);
         MVM_jit_log(tc, "Build tree out of: [");

src/jit/expr_ops.h

@@ -13,6 +13,8 @@
    src/jit/expr.h, so keep it in order!
 */
 #define MVM_JIT_EXPR_OPS(_) \
+    /* invalid operator */ \
+    _(NOOP, 0, 0, VOID, NO_CAST), \
     /* memory access */ \
     _(LOAD, 1, 1, REG, NO_CAST),   \
     _(STORE, 2, 1, VOID, NO_CAST), \