LCNF ReduceArity should also work for non-fix unused arguments.

Prerequisites

• Check that your issue is not already filed:
  https://github.com/leanprover/lean4/issues
• Reduce the issue to a minimal, self-contained, reproducible test case.
  Avoid dependencies to Mathlib or Batteries.
• Test your test case against the latest nightly release, for example on
  https://live.lean-lang.org/#project=lean-nightly
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Description

In code such as

set_option trace.Compiler.reduceArity true in
def simple (n p : Nat) : Nat :=
  match n with
  | 0 => 0
  | n+1 => 1 + simple n (n + p)

the p argument is in fact unused. However, as p is used in a computation to compute the next p, the compiler does currently not detect this (it only operates on fixed unused arguments) and thus fails to remove the argument.

Context

Reported by @ineol

Steps to Reproduce

Expected behavior: The p argument should be removed (and then latter the computation eliminated by the simplifier.

Actual behavior: p is left untouched.

Versions

Lean 4.30.0-rc1
Target: x86_64-unknown-linux-gnu

Additional Information

Impact

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Start with the minimal reproducer in the issue and run it with trace.Compiler.reduceArity enabled to observe the current behavior. Then locate the compiler's ReduceArity implementation and determine how the non-fixed unused argument is handled. Done means the p argument is removed and the subsequent computation can be eliminated by simplification.