cprop: Fix -fcompare-debug bug in constprop_register [PR100148]

The following testcase shows different behavior between -g and -g0
in constprop_register, if a flags register setter is separated
from a conditional jump using those flags with -g by a DEBUG_INSN.
As it uses just NEXT_INSN, for -g it will look at the DEBUG_INSN which is
not a conditional jump, while otherwise it would look at the conditional
jump and call cprop_jump.

2021-04-21  Jakub Jelinek  <jakub@redhat.com>

	PR rtl-optimization/100148
	* cprop.c (constprop_register): Use next_nondebug_insn instead of
	NEXT_INSN.

	* g++.dg/opt/pr100148.C: New test.

--- gcc/cprop.c
+++ gcc/cprop.c
@@ -1007,16 +1007,18 @@ static int
 constprop_register (rtx from, rtx src, rtx_insn *insn)
 {
   rtx sset;
+  rtx_insn *next_insn;
 
   /* Check for reg or cc0 setting instructions followed by
      conditional branch instructions first.  */
   if ((sset = single_set (insn)) != NULL
-      && NEXT_INSN (insn)
-      && any_condjump_p (NEXT_INSN (insn)) && onlyjump_p (NEXT_INSN (insn)))
+      && (next_insn = next_nondebug_insn (insn)) != NULL
+      && any_condjump_p (next_insn)
+      && onlyjump_p (next_insn))
     {
       rtx dest = SET_DEST (sset);
       if ((REG_P (dest) || CC0_P (dest))
-	  && cprop_jump (BLOCK_FOR_INSN (insn), insn, NEXT_INSN (insn),
+	  && cprop_jump (BLOCK_FOR_INSN (insn), insn, next_insn,
 			 from, src))
 	return 1;
     }
--- gcc/testsuite/g++.dg/opt/pr100148.C
+++ gcc/testsuite/g++.dg/opt/pr100148.C
@@ -0,0 +1,27 @@
+// PR rtl-optimization/100148
+// { dg-do compile }
+// { dg-options "-O2 -fno-dce -fno-tree-dce -fno-tree-dominator-opts -fno-tree-sink -fcompare-debug" }
+
+int i;
+enum E { } e, ee;
+
+bool
+baz (int)
+{
+  return ee;
+}
+
+bool bar ();
+bool a, b;
+
+void
+foo ()
+{
+  switch (ee)
+    {
+    case 0:
+      e = E (a ? : i);
+    case 1:
+      !(b || baz (0) && bar ());
+    }
+}