(combine_instructions): Initialize undobuf in the event

`gen_rtx_combine' is called before `try_combine'.

From-SVN: r3359

gcc/combine.c

@@ -406,6 +406,7 @@ combine_instructions (f, nregs)
   combine_merges = 0;
   combine_extras = 0;
   combine_successes = 0;
+  undobuf.num_undo = previous_num_undos = 0;
 
   combine_max_regno = nregs;