[PATCH] Fix PR 28807 - wrong code with may_alias and structs

[Andrew Pinski]

The problem here is that access_can_touch_variable would return false
for accesses via a base which has an aliasing set of 0 which is
incorrect.  Anyways this fixes the problem by checking the aliasing set
to make sure that does not happen.

I added a testcase to the testsuite for a possible missed optimization
which Daniel Berlin had caught with an earlier version of this patch.

OK? Bootstrapped and tested on i686-linux-gnu with no regressions.

Thanks,
Andrew Pinski

PS. The testcase for testing if this works fails at "-O3 -g" currently
but it is fails with an ICE which was also there before my patch.  I
filed the bug under PR 28834 and also noticed it is a regression.

ChangeLog:
	* tree-ssa-operands.c (access_can_touch_variable): Don't say
	the access through a base which has an alias set of 0 cannot
	touch the variable.

testsuite/ChangeLog:
	* gcc.c-torture/execute/mayalias-2.c: New test.
	* gcc.dg/tree-ssa/alias-13.c: New test.
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