cse_expr PanicException on main

[cmdlineluser]

Checks

• I have checked that this issue has not already been reported.
• I have confirmed this bug exists on the latest version of Polars.

Reproducible example

import polars as pl

lf = pl.LazyFrame({'x': 1, 'y': 5})

lf.with_columns(
    pl.all().exp() + pl.min_horizontal(pl.all().exp())
).collect()
# PanicException: called `Option::unwrap()` on a `None` value

Log output

thread '<unnamed>' panicked at crates/polars-plan/src/plans/optimizer/cse/cse_expr.rs:775:72:
called `Option::unwrap()` on a `None` value

Issue description

The query runs in 1.21.0 but currently panics on main for me.

.explain() also panics.

It runs with comm_subexpr_elim disabled.

lf.with_columns(
    pl.all().exp() + pl.min_horizontal(pl.all().exp())
).collect(comm_subexpr_elim=False)

# shape: (1, 2)
# ┌──────────┬────────────┐
# │ x        ┆ y          │
# │ ---      ┆ ---        │
# │ f64      ┆ f64        │
# ╞══════════╪════════════╡
# │ 5.436564 ┆ 151.131441 │
# └──────────┴────────────┘

Expected behavior

No panic.

Installed versions

main

[coastalwhite]

Pretty sure it is 6a3fc59. @ritchie46 can you have a look?