Simply execute:

  $ gcc -fdump-tree-all -fdump-rtl-all -save-temps -O3 -flto -g3 \
    reduction1.c reduction.c -o reduction && ./reduction
  ffffffff

and observe that the high 32 bits of the dpyinfo pointer in the saved
event are truncated, as would be expected from these expressions:

  ev_12 = MEM <unsigned int> [(union buffered_input_event *)&main_inev];

[...]

  MEM <unsigned int> [(union buffered_input_event *)&kbd_buffer] = ev_12;
  MEM <unsigned int> [(union buffered_input_event *)&kbd_buffer + 4B] = ev$ie$code_16;
  MEM <unsigned int> [(union buffered_input_event *)&kbd_buffer + 8B] = ev$ie$modifiers_1;
  MEM <long int> [(union buffered_input_event *)&kbd_buffer + 16B] = ev$ie$device_6;

With aggregate access expansion disabled, all is normal:

  $ gcc -fdump-tree-all -fdump-rtl-all -fno-tree-sra -save-temps \
    -O3 -flto -g3 reduction1.c reduction.c -o reduction && ./reduction
  ffffffffffffffff

In a word, tampering with optimizer options is tantamount to playing
with fire.  Don't do it, if your object is a functioning Emacs.