[PATCH] Implement the easy part of -fargument-noalias-global

[Andrew Pinski <pinskia at physics dot uc dot edu>]

The following patch implements the easiest part of 
-fargument-noalias-global,
which we can represent in the current tree aliasing infrastructure.

Say we have the following C code:
int f;
void link_error ();

void g(int *i)
{
  *i = 0;
  f = 1;
  if (*i != 0)
    link_error ();
}

With -fargument-noalias-global, we know that *i cannot point to f.  
This patch
enables that fact in may_alias.  So we should be able to remove the 
conditional.
In fact we already do it at the RTL level but since we have more 
aliasing
information at the tree level, this would enable for more optimizations 
which
we don't catch to the RTL level.

OK? Bootstrapped and tested on powerpc-darwin with no regressions.

Thanks,
Andrew Pinski

ChangeLog:
	* tree-ssa-alias.c (may_alias_p): If the variable is a global variable
	and the pointer is parameter and -fargument-noalias-global is used, 
then
	the pointer cannot alias the variable.

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