Patch to support mark_constant_function marking pure functions

[John Wehle]

>	(global_reg_mentioned_p): New function.
>	(nonlocal_referenced_p): Likewise.
>	(nonlocal_set_p): Likewise.
>
> I think these should be in rtlanal.c and non-static.

non_local_referenced_p and nonlocal_set_p call nonlocal_mentioned_p
which requires information only present during alias analysis (
find_base_term is used which is a static function already in alias.c).

In addition global_reg_mentioned_p and nonlocal_mentioned_p use
global_regs.  There is a comment in reg_set_p to the affect that
call_used_regs can't be used in rtlanal.c due to genattrtab.  I
imagine that the same comment applies to global_regs.

-- John
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