(previous_num_undos): Deleted variable.

(previous_num_undos): Deleted variable.

(MAX_UNDO): Deleted macro.
(struct undo): New field, next.
(struct undobuf): Deleted num_undos and undo.
New fields undos, frees, and previous_undos.
(SUBST, SUBST_INT): Rework to allocate memory and chain undo entries.
(combine_instructions): Initialize undobuf.{undos,previous_undos},
not undobuf.num_undo and previous_num_undos.
(try_combine): Likewise.
(undo_all, gen_rtx_combine): Rework to use new data structures.

From-SVN: r12372