[tree-ssa] dead const/pure/alloca call removal

[Nathanael Nerode]

+ /* Return true if CALL can not be removed in case its return value is 
dead.
+    In addition to const and pure functions we may elliminate alloca
+    builtin too.  */

Actually, I would write the comment this way, for maximum clarity:

"Return true if CALL cannot be removed even when its return value is 
dead.  (Return false if the return value is the only significant part of 
the call, such as when a pure function is being called.)"