[tree-ssa] Fix gcc.dg/tree-ssa/20030530-2.c

[law@redhat.com]

In message <wvln0gofx5n.fsf@prospero.boston.redhat.com>, Jason Merrill writes:
 >> Also, don't we recognize functions whose calls can be elided if they're
 >> called with the same arguments repeatedly?  (I forget the technical term
 >> :)
 >
 >"pure".  Not yet, because of TREE_SIDE_EFFECTS.  Perhaps we shouldn't set
 >TREE_SIDE_EFFECTS on calls to pure functions.
Actually, it's "const" -- "pure" functions are allowed to access global
memory.  And, yes, it seems to me the best way to get this optimization
is to not set TREE_SIDE_EFFECTS on "const" functions.

Jeff