[IRA] Segfault in ira_costs.c:find_costs_and_classes for AVR target

[Senthil Kumar Selvaraj]

Hi,

  gcc.c-torture/compile/pr34856.c and a couple of other tests segfault 
  for the AVR target. Looking at the code, I found that the
  x_ira_register_move_cost array[TImode] is NULL, while the code goes on
  to dereference it (ira_register_move_cost
  [ALLOCNO_MODE (a)][best][aclass] at line 1832).

  I looked at the code that populates the array, and found that it does
  explicitly allow the array to have NULL entries for certain modes,
  atleast during initialization.

  I'm not really sure how this should be fixed - is NULL valid or is the
  AVR target violating some invariant?

Regards
Senthil