This backport contains 1 patch from gcc main stream tree. ^M
The commit id of these patchs list as following in the order of time.

0001-Make-ifcvt-clean-up-dead-comparisons.patch
f1f10541903b082d27114db38947fb31f5364bcc

diff -Nurp a/gcc/basic-block.h b/gcc/basic-block.h
--- a/gcc/basic-block.h	2020-09-12 14:42:34.268000000 +0800
+++ b/gcc/basic-block.h	2020-09-12 14:42:48.448000000 +0800
@@ -507,6 +507,8 @@ ei_cond (edge_iterator ei, edge *p)
 #define CLEANUP_CFGLAYOUT	32	/* Do cleanup in cfglayout mode.  */
 #define CLEANUP_CFG_CHANGED	64      /* The caller changed the CFG.  */
 #define CLEANUP_NO_PARTITIONING	128     /* Do not try to fix partitions.  */
+#define CLEANUP_FORCE_FAST_DCE	0x100	/* Force run_fast_dce to be called
+					   at least once.  */
 
 /* Return true if BB is in a transaction.  */
 
diff -Nurp a/gcc/cfgcleanup.c b/gcc/cfgcleanup.c
--- a/gcc/cfgcleanup.c	2020-09-12 14:42:34.292000000 +0800
+++ b/gcc/cfgcleanup.c	2020-09-12 14:42:48.448000000 +0800
@@ -3197,7 +3197,10 @@ cleanup_cfg (int mode)
 	      && !delete_trivially_dead_insns (get_insns (), max_reg_num ()))
 	    break;
 	  if ((mode & CLEANUP_CROSSJUMP) && crossjumps_occurred)
-	    run_fast_dce ();
+	    {
+	      run_fast_dce ();
+	      mode &= ~CLEANUP_FORCE_FAST_DCE;
+	    }
 	}
       else
 	break;
@@ -3206,6 +3209,9 @@ cleanup_cfg (int mode)
   if (mode & CLEANUP_CROSSJUMP)
     remove_fake_exit_edges ();
 
+  if (mode & CLEANUP_FORCE_FAST_DCE)
+    run_fast_dce ();
+
   /* Don't call delete_dead_jumptables in cfglayout mode, because
      that function assumes that jump tables are in the insns stream.
      But we also don't _have_ to delete dead jumptables in cfglayout
diff -Nurp a/gcc/ifcvt.c b/gcc/ifcvt.c
--- a/gcc/ifcvt.c	2020-09-12 14:42:34.300000000 +0800
+++ b/gcc/ifcvt.c	2020-09-12 14:42:48.448000000 +0800
@@ -5457,6 +5457,8 @@ if_convert (bool after_combine)
 static unsigned int
 rest_of_handle_if_conversion (void)
 {
+  int flags = 0;
+
   if (flag_if_conversion)
     {
       if (dump_file)
@@ -5466,9 +5468,12 @@ rest_of_handle_if_conversion (void)
 	}
       cleanup_cfg (CLEANUP_EXPENSIVE);
       if_convert (false);
+      if (num_updated_if_blocks)
+	/* Get rid of any dead CC-related instructions.  */
+	flags |= CLEANUP_FORCE_FAST_DCE;
     }
 
-  cleanup_cfg (0);
+  cleanup_cfg (flags);
   return 0;
 }