Uninitialized padding in struct _dep.

Uninitialized padding in struct _dep.

In struct _dep, there is an implicit padding of 4bits.  This
bit-field padding is uninitialized when init_dep_1 is being called.
This means we access uninitialized memory but never use it for
anything.  Adding an unused bit-field field and initializing it
in init_dep_1 will improve code generation also as we initialize
the whole 32bits now rather than just part of it.

ChangeLog:
* sched-int.h (_dep): Add unused bit-field field for the padding.
* sched-deps.c (init_dep_1): Init unused field.