`select` not optimized when condition is true and ifFalse arm isn't pure

Consider the following expression:

(select
	(i32.div_u (i32.const 3) (i32.const 0))
	(i32.const 2)
	(i32.const 0)
)

Binaryen will try to eliminate the select statement when its condition is constant.

The div_u might cause a trap though, so under safe optimizations (without -tnh or -iit) it needs to be preserved.

Binaryen outputs both arms, and drops the value we don't need:

(drop (i32.div_u (i32.const 3) (i32.const 0)))
(i32.const 2)

Now let's look at the opposite case:

(select
	(i32.const 2)
	(i32.div_u (i32.const 3) (i32.const 0))
	(i32.const 1)
)

Surely it will use the same trick, right? Output both arms, drop the one you don't need:

(i32.const 2)
(drop (i32.div_u (i32.const 3) (i32.const 0)) )

Except it doesn't! For some reason it completely gives up here, and keeps the select and its condition as-is.

The code mentions something about needing to "reverse the order using a temp local" but that makes no sense to me.

Start with the select-optimization logic in src/passes/OptimizeInstructions.cpp at lines 2685-2704, especially the comment about reversing evaluation order with a temporary local. Compare the two constant-condition cases in the issue and verify that safe optimizations preserve a potentially trapping arm while still eliminating the unnecessary select when possible.