amd64: Prevent initial IncSP from disappearing in corner cases.

This is the same correction as for ia32.
First construct all epilogues, then the prologue.
Check there, whether the IncSP has no users and add a keep edge in that case.

ir/be/amd64/bearch_amd64.c

@@ -577,7 +577,7 @@ static void introduce_epilogue(ir_node *ret)
 	}
 }
 
-static void introduce_prologue_epilogue(ir_graph *irg)
+static void introduce_prologue(ir_graph *const irg)
 {
 	const arch_register_t *sp         = &amd64_registers[REG_RSP];
 	const arch_register_t *bp         = &amd64_registers[REG_RBP];
@@ -611,7 +611,7 @@ static void introduce_prologue_epilogue(ir_graph *irg)
 		sched_add_after(curr_sp, incsp);
 
 		/* make sure the initial IncSP is really used by someone */
-		if (get_irn_n_edges(incsp) <= 1) {
+		if (get_irn_n_edges(incsp) == 0) {
 			ir_node *in[] = { incsp };
 			ir_node *keep = be_new_Keep(block, 1, in);
 			sched_add_after(incsp, keep);
@@ -625,12 +625,17 @@ static void introduce_prologue_epilogue(ir_graph *irg)
 			sched_add_after(start, incsp);
 		}
 	}
+}
 
+static void introduce_prologue_epilogue(ir_graph *irg)
+{
 	/* introduce epilogue for every return node */
 	foreach_irn_in(get_irg_end_block(irg), i, ret) {
 		assert(is_amd64_ret(ret));
 		introduce_epilogue(ret);
 	}
+
+	introduce_prologue(irg);
 }
 
 /**