PR101260 regcprop: Add mode change check for copy reg

PR101260 regcprop: Add mode change check for copy reg

When propagating a multi-word register into an access with a smaller
mode the can_change_mode backend hook is already consulted for the
original register.  This however is also required for the intermediate
copy in copy_regno which might use a different register class.

gcc/ChangeLog:

	PR rtl-optimization/101260
	* regcprop.cc (maybe_mode_change): Invoke mode_change_ok also for
	copy_regno.

gcc/testsuite/ChangeLog:

	PR rtl-optimization/101260
	* gcc.target/s390/pr101260.c: New testcase.