StackIR: Optimize away a drop before an unreachable

[kripken]

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

[tlively]

In principle this could go even further. We could remove both drops here:

(drop (call $foo) ) (drop (call $bar) ) (unreachable) 

[kripken]

Yeah, that looks possible too, if we analyze the stack backwards. I'll add a TODO.

[tlively]

Another opportunity here would be to remove drops preceding branches:

(drop ;; can be removed (call $foo) ) (br $l ;; could be any control flow transfer (call $bar) )