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cc1: Remove wrong cast simpleifications

Casts cannot be collapse, because every cast can modify
the value (truncating values). The correct behaviour
is to call foldcast() and let it to do at compilation
time the work that should be done at execution time.

--- a/src/cmd/cc/cc1/fold.c

+++ b/src/cmd/cc/cc1/fold.c

@@ -318,7 +318,7 @@

 	Type *newtp = np->type, *oldtp = l->type;

 	Symbol aux, *sym, *osym = l->sym;

-	if (!(l->flags & NCONST))

+	if ((l->flags & NCONST) == 0)

 		return np;

 	switch (newtp->op) {

@@ -381,14 +381,7 @@

 		freetree(np);

 		return l;

 	case OCAST:

-		if (op != OCAST)

-			return foldcast(np, l);

-		/* TODO: This is wrong: (float)(int) 7.2 */

-		DBG("FOLD unary collapse %d", np->op);

-		np->left = l->left;

-		l->left = NULL;

-		freetree(l);

-		return np;

+		return foldcast(np, l);

 	case OSNEG:

 	case OCPL:

 		if (op != np->op)