Fix segfault on elaboration of empty 1-element array at -O

This is a rather obscure case where the elaboration of an empty array
whose base type is an array type of length at most 1 goes awry when
the code is compiled with optimization.

gcc/ada/ChangeLog:
	* gcc-interface/trans.c (can_be_lower_p): Remove.
	(Regular_Loop_to_gnu): Add ENTRY_COND unconditionally if
	BOTTOM_COND is non-zero.

gcc/testsuite/ChangeLog:
	* gnat.dg/opt89.adb: New test.