Internal consistency check failure (`{{x,_},not_live}` at `allocate_heap`)

[fixeria]

Describe the bug
Compiling the module below fails with an internal consistency check failure on OTP 29.0.3. The same module compiles fine on OTP 29.0.1, so this appears to be a regression introduced in one of the 29.0.2/29.0.3 patch releases.

The trigger seems to be two chained setelement/3 calls on a tuple of unknown shape, where the first setelement stores a freshly built cons cell. Compiling with +no_type_opt (or +no_ssa_opt) makes the error go away, which points at the SSA type-optimization pass.

Originally observed while building the gen_netlink application (function netlink:prepare_request/2).

To Reproduce
Compile the following module with plain erlc repro.erl:

-module(repro).
-export([prepare_request/2]).

prepare_request(Msg0, Seq) ->
    Flags = element(3, Msg0),
    Msg1 = setelement(3, Msg0, [request | Flags]),
    setelement(4, Msg1, Seq).

Output on OTP 29.0.3:

repro:1: function prepare_request/2+7:
  Internal consistency check failed - please report this bug.
  Instruction: {allocate_heap,1,2,4}
  Error:       {{x,2},not_live}:

Notes:

• Compiles cleanly on OTP 29.0.1.
• erlc +no_type_opt repro.erl and erlc +no_ssa_opt repro.erl both succeed on 29.0.3.
• Swapping the order of the two setelement/3 calls (setting element 4 first, then element 3) also avoids the failure:

prepare_request(Msg0, Seq) ->
    Flags = element(3, Msg0),
    Msg1 = setelement(4, Msg0, Seq),
    setelement(3, Msg1, [request | Flags]).

Expected behavior
The module compiles without errors, as it does on OTP 29.0.1.

Affected versions

• Fails: OTP 29.0.3 (Erlang/OTP 29 [erts-17.0.3])
• Works: OTP 29.0.1 (Erlang/OTP 29 [erts-17.0.1])

OTP 29.0.2 not tested.

Additional context

• OS: Debian GNU/Linux forky/sid (x86_64), OTP 29.0.3 from Debian packages (erlang 1:29.0.3+dfsg-1).
• Emulator: [64-bit] [smp:16:16] [jit:ns]