combine: Fix PR84780 (more LOG_LINKS trouble)

There still are situations where we have stale LOG_LINKS.  This causes
combine to try two-insn combinations I2->I3 where the register set by
I2 is used before I3 as well.  Not good.

This patch fixes it by checking for this situation in can_combine_p
(similar to what we already do for three and four insn combinations).

From-SVN: r258452

gcc/ChangeLog

+2018-03-12  Segher Boessenkool  <segher@kernel.crashing.org>
+
+	PR rtl-optimization/84169
+	PR rtl-optimization/84780
+	* combine.c (can_combine_p): Check for a 2-insn combination whether
+	the destination register is used between the two insns, too.
+
 2018-03-12  Richard Biener  <rguenther@suse.de>
 
 	PR tree-optimization/84803

gcc/combine.c

@@ -1990,6 +1990,7 @@ can_combine_p (rtx_insn *insn, rtx_insn *i3, rtx_insn *pred ATTRIBUTE_UNUSED,
 	       && (reg_used_between_p (dest, succ2, i3)
 		   || reg_used_between_p (dest, succ, succ2)))
 	      || (!succ2 && succ && reg_used_between_p (dest, succ, i3))
+	      || (!succ2 && !succ && reg_used_between_p (dest, insn, i3))
 	      || (succ
 		  /* SUCC and SUCC2 can be split halves from a PARALLEL; in
 		     that case SUCC is not in the insn stream, so use SUCC2