CRIS: peephole2 an add into two addq or subq

CRIS: peephole2 an add into two addq or subq

Unfortunately, doesn't cause a performance improvement for coremark,
but happens a few times in newlib, just enough to affect coremark
0.01% by size (or 4 bytes, and three cycles (__fwalk_sglue and
__vfiprintf_r each two bytes).

gcc:
	* config/cris/cris.md (splitop): Add PLUS.
	* config/cris/cris.cc (cris_split_constant): Also handle
	PLUS when a split into two insns may be useful.

gcc/testsuite:
	* gcc.target/cris/peep2-addsplit1.c: New test.