Allow propagations from inner loop to outer loop.

NULL is considered as an outer loop of any other loop.

gcc/ChangeLog:

	PR rtl-optimization/103750
	* fwprop.c (forward_propagate_into): Allow propagations from
	inner loop to outer loop.

gcc/testsuite/ChangeLog:

	* g++.target/i386/pr103750-fwprop-1.C: New test.