Using leaf_function_p in a backend is dangerous as it incorrectly returns

Using leaf_function_p in a backend is dangerous as it incorrectly returns
false if it is called while in a sequence (for example during prolog/epilog
generation).  Replace all uses with crtl->is_leaf as this is now initialized
early enough in ira.c.  This typically causes no code generation differences
unless there was a bug due to leaf_function_p returning the wrong value.

    gcc/
        * config/arm/arm.h (TARGET_BACKTRACE): Use crtl->is_leaf.
        * config/arm/arm.c (arm_option_check_internal): Improve comment.
        (thumb_force_lr_save): Use crtl->is_leaf.
        (arm_get_frame_offsets): Remove comment.  Use crtl->is_leaf.
        (thumb_far_jump_used_p): Remove comment.
        (arm_frame_pointer_required): Use crtl->is_leaf.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@243720 138bc75d-0d04-0410-961f-82ee72b054a4