StackIR: Optimize away a drop before an unreachable (#6719)

Anything else right before an unreachable is removed by the main DCE
pass anyhow, but because of the structured form of BinaryenIR we can't remove
a drop. That is, this is the difference between

(i32.eqz
  (i32.const 42)
  (unreachable)
)

and

(drop
  (call $foo)
)
(unreachable)

In both cases the unreachable is preceded by something we don't need,
but in the latter case it must remain in BinaryenIR for validation.

To optimize this, add a rule in StackIR.

Fixes #6715

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