tree-optimization/120043 - bogus conditional store elimination

tree-optimization/120043 - bogus conditional store elimination

The following fixes conditional store elimination to properly
check for conditional stores to readonly memory which we can
obviously not store to unconditionally.  The tree_could_trap_p
predicate used is only considering rvalues and the chosen
approach mimics that of loop store motion.

	PR tree-optimization/120043
	* tree-ssa-phiopt.cc (cond_store_replacement): Check
	whether the store is to readonly memory.

	* gcc.dg/torture/pr120043.c: New testcase.