df: make df_problem instances "const"

The various struct df_problem instances are constant data; mark them
as such.

gcc/ChangeLog:
	* df-core.c (df_add_problem): Make the problem param be const.
	(df_remove_problem): Make local "problem" be const.
	* df-problems.c (problem_RD): Make const.
	(problem_LR): Likewise.
	(problem_LIVE): Likewise.
	(problem_MIR): Likewise.
	(problem_CHAIN): Likewise.
	(problem_WORD_LR): Likewise.
	(problem_NOTE): Likewise.
	(problem_MD): Likewise.
	* df-scan.c (problem_SCAN): Likewise.
	* df.h (struct df_problem): Make field "dependent_problem" be
	const.
	(struct dataflow): Likewise for field "problem".
	(df_add_problem): Make param const.

From-SVN: r235524