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.