rs6000: build constant via li/lis;rldic

rs6000: build constant via li/lis;rldic

This patch checks if a constant is possible to be built by "li;rldic".
Only need to take care of "negative li", other forms do not need to check.
For example, "negative lis" is just a "negative li" with an additional shift.

gcc/ChangeLog:

	* config/rs6000/rs6000.cc (can_be_built_by_li_and_rldic): New function.
	(rs6000_emit_set_long_const): Call can_be_built_by_li_and_rldic.

gcc/testsuite/ChangeLog:

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