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Allow folding initializers for constants.

    Todo, prevent cyclic initializations. Eg:

        const x = y
        const y = x

--- a/6/simp.c

+++ b/6/simp.c

@@ -1411,7 +1411,7 @@

             printf("FOLD FROM ----------\n");

             dump(s->stmts[i], stdout);

         }

-        s->stmts[i] = fold(s->stmts[i]);

+        s->stmts[i] = fold(s->stmts[i], 0);

         if (debugopt['f']) {

             printf("TO ------------\n");

             dump(s->stmts[i], stdout);

@@ -1459,7 +1459,7 @@

 {

     Node *e;

-    dcl->decl.init = fold(dcl->decl.init);;

+    dcl->decl.init = fold(dcl->decl.init, 1);;

     e = dcl->decl.init;

     if (e && exprop(e) == Olit) {

         if (e->expr.args[0]->lit.littype == Lfunc)

--- a/mi/fold.c

+++ b/mi/fold.c

@@ -45,7 +45,7 @@

     return n;

 }

-Node *fold(Node *n)

+Node *fold(Node *n, int foldvar)

 {

     Node **args, *r;

     vlong a, b;

@@ -59,10 +59,11 @@

     r = NULL;

     args = n->expr.args;

     for (i = 0; i < n->expr.nargs; i++)

-        args[i] = fold(args[i]);

+        args[i] = fold(args[i], foldvar);

     switch (exprop(n)) {

         case Ovar:

-            /* FIXME: chase small consts */

+            if (foldvar && decls[n->expr.did]->decl.isconst)

+                r = fold(decls[n->expr.did]->decl.init, foldvar);

             break;

         case Oadd:

             /* x + 0 = 0 */

--- a/mi/opt.h

+++ b/mi/opt.h

@@ -25,7 +25,7 @@

 };

 /* expression folding */

-Node *fold(Node *n);

+Node *fold(Node *n, int foldvar);

 /* Takes a reduced block, and returns a flow graph. */

 Cfg *mkcfg(Node **nl, size_t nn);

 void dumpcfg(Cfg *c, FILE *fd);