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
