[wasm-split] Don't split trapping globals

[aheejin]

Trapping globals, which became possible with the custom-descriptor proposal, should stay in the primary module because we have to preserve their trapping behavior upon instantiation.

Also, note that we remove unused globals here:

binaryen/src/ir/module-splitting.cpp

Lines 896 to 903 in 17a9078

	if (!inPrimary && usingSecondaries.empty()) {
	// It's not used anywhere, so delete it. Unlike other unused module items
	// (memories, tables, and tags) that can just sit in the primary module
	// and later be DCE'ed by another pass, we should remove it here, because
	// an unused global can contain an initializer that refers to another
	// global that will be moved to a secondary module, like
	// (global $unused i32 (global.get $a)) // $a is moved to a secondary
	globalsToRemove.push_back(global->name);

But trapping globals shouldn't be removed even when they are unused to preserve the trapping behavior.

In case someone wants to split modules assuming traps never happen, this also adds --traps-never-happen option to wasm-split, in which case we can freely split or remove trapping globals.