Fix PR 106560: Another ICE after conflicting types of redeclaration

This another one of these ICE after error issues with the
gimplifier and a fallout from r12-3278-g823685221de986af.
The problem here is gimplify_modify_expr does not
check if either from or to was an error operand.
This adds the check and fixes the ICE.

OK? Bootstrapped and tested on x86_64-linux-gnu with no regressions.

gcc/ChangeLog:

	* gimplify.cc (gimplify_modify_expr): If
	either *from_p or *to_p were error_operand
	return early.

gcc/testsuite/ChangeLog:

	* gcc.dg/redecl-23.c: New test.
	* gcc.dg/redecl-24.c: New test.
	* gcc.dg/redecl-25.c: New test.

gcc/gimplify.cc

@@ -6054,6 +6054,9 @@ gimplify_modify_expr (tree *expr_p, gimple_seq *pre_p, gimple_seq *post_p,
   location_t loc = EXPR_LOCATION (*expr_p);
   gimple_stmt_iterator gsi;
 
+  if (error_operand_p (*from_p) || error_operand_p (*to_p))
+    return GS_ERROR;
+
   gcc_assert (TREE_CODE (*expr_p) == MODIFY_EXPR
 	      || TREE_CODE (*expr_p) == INIT_EXPR);
 

gcc/testsuite/gcc.dg/redecl-23.c

@@ -0,0 +1,6 @@
+/* We used to ICE in the gimplifier, PR 106560. */
+/* { dg-do compile } */
+/* { dg-options "-w" } */
+void **a; /* { dg-note "" } */
+void b() { void **c = a; }
+a; /* { dg-error "" } */

gcc/testsuite/gcc.dg/redecl-24.c

@@ -0,0 +1,6 @@
+/* We used to ICE in the gimplifier, PR 106560 */
+/* { dg-do compile } */
+/* { dg-options "-w" } */
+void **a, **b; /* { dg-note "" } */
+c(){b = a;}  
+a = /* { dg-error "" } */

gcc/testsuite/gcc.dg/redecl-25.c

@@ -0,0 +1,9 @@
+/* We used to ICE in the gimplifier, PR 106560 */
+/* { dg-do compile } */
+/* { dg-options "-w" } */
+void **a; /* { dg-note "" } */
+void b() {
+  void **c; 
+    c = a /* { dg-error "" } */
+}
+a; /* { dg-error "" } */