arc: Fix for loop end detection

We use a negative ID number to link together the doloop_begin and
doloop_end instructions.  This negative ID number is setup within
doloop_begin, at this point the ID is stored into the loop end
instruction (doloop_end_i) and placed into the doloop_begin_i
instruction.

In arc.c (arc_reorg) we extract the ID from the doloop_end_i
instruction in order to find the matching doloop_begin_i instruction,
though the ID is only used in some cases.

Currently in arc_reorg when we extract the ID we negate it.  This
negation is invalid.  The ID stored in both doloop_end_i and
doloop_begin_i is already negative, the negation in arc_reorg means
that if we need to use the ID to find the doloop_begin_i then we will
never find it (as the IDs will never match).

This commit removes the unneeded negation, moves the extraction of the
ID into a more appropriately scoped block and adds a new test for this
issue.

gcc/ChangeLog:

	* config/arc/arc.c (arc_reorg): Move loop_end_id into a more local
	block, and do not negate it, the stored id is already negative.

gcc/testsuite/ChangeLog:

	* gcc.target/arc/loop-1.c: New file.


Co-Authored-By: Guy Benyei <guybe@mellanox.com>

From-SVN: r246933