glsl: fix downcasting addresses to wrong object types

This fixes several downcasting of address to object types when the
original object types were either different or invalid.

This has been detected throught Undefined Behaviour Sanitizer (UBSan).
An example of such issue were:

`downcast of address 0x55559c0cbcc0 which does not point to an object of
type 'ir_variable' 0x55559c0cbcc0: note: object is of type 'ir_constant'

Reviewed-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Signed-off-by: Juan A. Suarez Romero <jasuarez@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/29772>

src/compiler/glsl/opt_dead_code_local.cpp

@@ -303,7 +303,8 @@ dead_code_local_basic_block(ir_instruction *first,
 			     ir_instruction *last,
 			     void *data)
 {
-   ir_instruction *ir, *ir_next;
+   ir_instruction *ir;
+   exec_node *node, *node_next;
    /* List of avaialble_copy */
    exec_list assignments;
    bool *out_progress = (bool *)data;
@@ -313,8 +314,9 @@ dead_code_local_basic_block(ir_instruction *first,
    linear_ctx *lin_ctx = linear_context(ctx);
 
    /* Safe looping, since process_assignment */
-   for (ir = first, ir_next = (ir_instruction *)first->next;;
-	ir = ir_next, ir_next = (ir_instruction *)ir->next) {
+   for (node = first, node_next = first->next;;
+	node = node_next, node_next = node->next) {
+      ir = (ir_instruction *) node;
       ir_assignment *ir_assign = ir->as_assignment();
 
       if (debug) {