rs6000: Define a pattern for mffscrni. If the RN is a constant, it can call gen_rs60...

rs6000: Define a pattern for mffscrni.  If the RN is a constant, it can call gen_rs6000_mffscrni directly.

gcc/
	* config/rs6000/rs6000.md (rs6000_mffscrni): Define.
	(rs6000_set_fpscr_rn): Change the type of operand[0] from DI to SI.
	Call gen_rs6000_mffscrni when operand[0] is a const_0_to_3_operand.

gcc/testsuite/
	* gcc.target/powerpc/mffscrni_p9.c: New testcase for mffscrni.
	* gcc.target/powerpc/test_fpscr_rn_builtin.c: Test mffscrn and mffscrni
	separately.