[5/9] Create sensible dummy registers

[Eric Botcazou]

> Some pieces of code create a temporary REG or MEM and only fill it
> in later when they're testing the cost of a particular rtx.  This patch
> makes sure that even the dummy REG or MEM is valid, rather than force
> the gen_* code to handle garbage values.
> 
> 
> gcc/
> 	* caller-save.c (init_caller_save): Use word_mode and
> 	FIRST_PSEUDO_REGISTER when creating temporary rtxes.
> 	* expr.c (init_expr_target): Likewise.
> 	* ira.c (setup_prohibited_mode_move_regs): Likewise.
> 	* postreload.c (reload_cse_regs_1): Likewise.

Isn't LAST_VIRTUAL_REGISTER + 1 the canonical regno to be used in this case?

-- 
Eric Botcazou