A SET operation that writes memory may have the same value as an earlier 
store but if the alias sets of the new and earlier store do not conflict 
then the set is not truly redundant.  This can happen, for example, if 
objects of different types share a stack slot.

To fix this we define a new function in cselib that first checks for 
equality and if that is successful then finds the earlier store in the 
value history and checks the alias sets.

The routine is used in two places elsewhere in the compiler.  Firstly
in cfgcleanup and secondly in postreload.

gcc/ChangeLog:
	* cselib.h (cselib_redundant_set_p): Declare.
	* cselib.cc: Include alias.h
	(cselib_redundant_set_p): New function.
	* cfgcleanup.cc: (mark_effect): Use cselib_redundant_set_p instead
	of rtx_equal_for_cselib_p.
	* postreload.c (reload_cse_simplify): Use cselib_redundant_set_p.
	(reload_cse_noop_set_p): Delete.