combine: Don't combine if I2 does not change

In some cases combine will "combine" an I2 and I3, but end up putting
exactly the same thing back as I2 as was there before.  This is never
progress, so we shouldn't do it, it will lead to oscillating behaviour
and the like.

If we want to canonicalise things, that's fine, but this is not the
way to do it.

2024-03-27  Segher Boessenkool  <segher@kernel.crashing.org>

	PR rtl-optimization/101523
	* combine.cc (try_combine): Don't do a 2-insn combination if
	it does not in fact change I2.

gcc/combine.cc

@@ -4186,6 +4186,17 @@ try_combine (rtx_insn *i3, rtx_insn *i2, rtx_insn *i1, rtx_insn *i0,
       adjust_for_new_dest (i3);
     }
 
+  /* If I2 didn't change, this is not a combination (but a simplification or
+     canonicalisation with context), which should not be done here.  Doing
+     it here explodes the algorithm.  Don't.  */
+  if (rtx_equal_p (newi2pat, PATTERN (i2)))
+    {
+      if (dump_file)
+	fprintf (dump_file, "i2 didn't change, not doing this\n");
+      undo_all ();
+      return 0;
+    }
+
   /* We now know that we can do this combination.  Merge the insns and
      update the status of registers and LOG_LINKS.  */