loop-manip: Avoid -fcompare-debug issues in create_iv [PR94285]

loop-manip: Avoid -fcompare-debug issues in create_iv [PR94285]

The following testcase FAILs with -fcompare-debug.  The problem is that
create_iv behaves differently when inserting after into an empty bb (in that
case sets location to goto_locus), or when inserting before gsi_end_p (i.e.
at the end of bb; in that case it will not set location, otherwise it will
set it to the location of next stmt).
This isn't -fcompare-debug friendly, because if inserting after and the
bb contains only debug stmts, then the location will not be set with -g
and will be with -g0, or when inserting before, the location might either
be set from the following debug stmt rather than some non-debug stmt after
that, or might not be set with -g0 if it is to be inserted at the end of bb,
while with -g would be set to location of debug stmt.

2020-03-24  Jakub Jelinek  <jakub@redhat.com>

	PR debug/94285
	* tree-ssa-loop-manip.c (create_iv): If after, set stmt location to
	e->goto_locus even if gsi_bb (*incr_pos) contains only debug stmts.
	If not after and at *incr_pos is a debug stmt, set stmt location to
	location of next non-debug stmt after it if any.

	* gfortran.dg/pr94285.f90: New test.