rs6000: build constant via li/lis;rldicl/rldicr

If a constant is possible left/right cleaned on a rotated value from
a negative value of "li/lis".  Then, using "li/lis ; rldicl/rldicr"
to build the constant.

gcc/ChangeLog:

	* config/rs6000/rs6000.cc (can_be_built_by_li_lis_and_rldicl): New
	function.
	(can_be_built_by_li_lis_and_rldicr): New function.
	(rs6000_emit_set_long_const): Call can_be_built_by_li_lis_and_rldicr and
	can_be_built_by_li_lis_and_rldicl.

gcc/testsuite/ChangeLog:

	* gcc.target/powerpc/const-build.c: Add more tests.