arc: Refurbish add.f combiner patterns

Refurbish add compare patterns: use 'r' constraint, fix identation,
and fix pattern to match 'if (a+b) { ... }' constructions.

gcc/

	* config/arc/arc.cc (arc_select_cc_mode): Match NEG code with
	the first operand.
	* config/arc/arc.md (addsi_compare): Make pattern canonical.
	(addsi_compare_2): Fix identation, constraint letters.
	(addsi_compare_3): Likewise.

gcc/testsuite/

	* gcc.target/arc/add_f-combine.c: New test.

Signed-off-by: Claudiu Zissulescu <claziss@gmail.com>