gimplify: Fix -fcompare-debug differences caused by gimplify_body [PR94281]

gimplify: Fix -fcompare-debug differences caused by gimplify_body [PR94281]

The following testcase FAILs, because gimplify_body adds a GIMPLE_NOP only
when there are no statements in the function and with -g there is a
DEBUG_BEGIN_STMT, so it doesn't add it and due to -fno-tree-dce that never
gets removed afterwards.  Similarly, if the body seq after gimplification
contains some DEBUG_BEGIN_STMTs plus a single gbind, then we could behave
differently between -g0 and -g, by using that gbind as the body in the -g0
case and not in the -g case.
This patch fixes that by ignoring DEBUG_BEGIN_STMTs (other debug stmts can't
appear at this point yet thankfully) during decisions and if we pick the
single gbind and there are DEBUG_BEGIN_STMTs next to it, it moves them into
the gbind.
While debugging this, I found also a bug in the gimple_seq_last_nondebug_stmt
function, for a seq that has a single non-DEBUG_BEGIN_STMT statement
followed by one or more DEBUG_BEGIN_STMTs it would return NULL rather than
the first statement.

2020-03-26  Jakub Jelinek  <jakub@redhat.com>

	PR debug/94281
	* gimple.h (gimple_seq_first_nondebug_stmt): New function.
	(gimple_seq_last_nondebug_stmt): Don't return NULL if seq contains
	a single non-debug stmt followed by one or more debug stmts.
	* gimplify.c (gimplify_body): Use gimple_seq_first_nondebug_stmt
	instead of gimple_seq_first_stmt, use gimple_seq_first_nondebug_stmt
	and gimple_seq_last_nondebug_stmt instead of gimple_seq_first and
	gimple_seq_last to check if outer_stmt gbind could be reused and
	if yes and it is surrounded by any debug stmts, move them into the
	gbind body.

	* g++.dg/debug/pr94281.C: New test.