Confirmed.

The following example is not strictly a seuqence of "if"s, but
do note that it is a real-world example extracted from function.c of GCC.

<L21>:;
  T.1674_116 = type_1->common.code;
  T.1675_117 = T.1674_116 == 18;
  T.1676_118 = T.1674_116 == 20;
  T.1677_119 = T.1675_117 || T.1676_118;
  if (T.1677_119) goto <L25>; else goto <L22>;

<L22>:;
  if (T.1674_116 == 21) goto <L25>; else goto <L23>;

<L23>:;
  if (T.1674_116 == 22) goto <L25>; else goto <L24>;

<L24>:;
  if (T.1674_116 == 19) goto <L25>; else goto <L26>;

<L25>:;
  return 1;

Here, we could do a simple range check like "if ((t - 18) <= 4)".

The example from the description field looks like this at tree level (optimized dump with 4.5.1). It takes until late in the rtl untill the duplicate call blocks are collapsed.
...
foo (int a)
{
<bb 2>:
  if (a_1(D) == 30)
    goto <bb 3>;
  else
    goto <bb 4>;

<bb 3>:
  bar (); [tail call]
  goto <bb 10>;

<bb 4>:
  if (a_1(D) == 31)
    goto <bb 5>;
  else
    goto <bb 6>;

<bb 5>:
  bar (); [tail call]
  goto <bb 10>;

<bb 6>:
  if (a_1(D) == 53)
    goto <bb 7>;
  else
    goto <bb 8>;

<bb 7>:
  bar (); [tail call]
  goto <bb 10>;

<bb 8>:
  if (a_1(D) == 23)
    goto <bb 9>;
  else
    goto <bb 10>;

<bb 9>:
  bar (); [tail call]

<bb 10>:
  return;
}
...

If the duplicate blocks would have been collapsed, it would look like this:
...
foo (int a)
{
<bb 2>:
  if (a_1(D) == 30)
    goto <bb 9>;
  else
    goto <bb 4>;

<bb 4>:
  if (a_1(D) == 31)
    goto <bb 9>;
  else
    goto <bb 6>;

<bb 6>:
  if (a_1(D) == 53)
    goto <bb 9>;
  else
    goto <bb 8>;

<bb 8>:
  if (a_1(D) == 23)
    goto <bb 9>;
  else
    goto <bb 10>;

<bb 9>:
  bar (); [tail call]

<bb 10>:
  return;
}
...
for this representation, the patch from PR 46935 comment 5 should work.

*** Bug 78484 has been marked as a duplicate of this bug. ***

*** Bug 87925 has been marked as a duplicate of this bug. ***

I'm working on the optimization.

The master branch has been updated by Martin Liska <marxin@gcc.gnu.org>:

https://gcc.gnu.org/g:03eb09292ef228d1d12b5168cdd748583b1f992a

commit r11-5605-g03eb09292ef228d1d12b5168cdd748583b1f992a
Author: Martin Liska <mliska@suse.cz>
Date:   Fri Aug 28 10:26:13 2020 +0200

    Add if-chain to switch conversion pass.
    
    gcc/ChangeLog:
    
            PR tree-optimization/14799
            PR ipa/88702
            * Makefile.in: Add gimple-if-to-switch.o.
            * dbgcnt.def (DEBUG_COUNTER): Add new debug counter.
            * passes.def: Include new pass_if_to_switch pass.
            * timevar.def (TV_TREE_IF_TO_SWITCH): New timevar.
            * tree-pass.h (make_pass_if_to_switch): New.
            * tree-ssa-reassoc.c (struct operand_entry): Move to the header.
            (dump_range_entry): Move to header file.
            (debug_range_entry): Likewise.
            (no_side_effect_bb): Make it global.
            * tree-switch-conversion.h (simple_cluster::simple_cluster):
            Add inline for couple of functions in order to prevent error
            about multiple defined symbols.
            * gimple-if-to-switch.cc: New file.
            * tree-ssa-reassoc.h: New file.
    
    gcc/testsuite/ChangeLog:
    
            PR tree-optimization/14799
            PR ipa/88702
            * gcc.dg/tree-ssa/pr96480.c: Disable if-to-switch conversion.
            * gcc.dg/tree-ssa/reassoc-32.c: Likewise.
            * g++.dg/tree-ssa/if-to-switch-1.C: New test.
            * gcc.dg/tree-ssa/if-to-switch-1.c: New test.
            * gcc.dg/tree-ssa/if-to-switch-2.c: New test.
            * gcc.dg/tree-ssa/if-to-switch-3.c: New test.
            * gcc.dg/tree-ssa/if-to-switch-4.c: New test.
            * gcc.dg/tree-ssa/if-to-switch-5.c: New test.
            * gcc.dg/tree-ssa/if-to-switch-6.c: New test.
            * gcc.dg/tree-ssa/if-to-switch-7.c: New test.
            * gcc.dg/tree-ssa/if-to-switch-8.c: New test.

Implemented in master.