Effects: Clarify trap effect meaning, and consider infinite loops to trap due to timeout (#5039)

I think this simplifies the logic behind what we consider to trap. Before we had kind of
a hack in visitLoop that now has a more clear reasoning behind it: we consider as
trapping things that trap in all VMs all the time, or will eventually. So a single allocation
doesn't trap, but an unbounded amount can, and an infinite loop is considered to
trap as well (a timeout in a VM will be hit eventually, somehow).

This means we cannot optimize way a trivial infinite loop with no effects in it,

while (1) {}

But we can optimize it out in trapsNeverHappen mode. In any event, such a loop
is not a realistic situation; an infinite loop with some other effect in it, like a call to
an import, will not be optimized out, of course.

Also clarify some other things regarding traps and trapsNeverHappen following
recent discussions in https://github.com/emscripten-core/emscripten/issues/17732
Specifically, TNH will never be allowed to remove calls to imports.

1 files changed, 3 insertions, 1 deletions

diff --git a/test/wasm2js/br_table_to_loop.2asm.js.opt b/test/wasm2js/br_table_to_loop.2asm.js.opt
index fb8e7f956..cac6dc28c 100644
--- a/test/wasm2js/br_table_to_loop.2asm.js.opt
+++ b/test/wasm2js/br_table_to_loop.2asm.js.opt
@@ -1,4 +1,6 @@
 
+function wasm2js_trap() { throw new Error('abort'); }
+
 function asmFunc(importObject) {
  var env = importObject.env || importObject;
  var Math_imul = Math.imul;
@@ -14,7 +16,7 @@ function asmFunc(importObject) {
  var nan = NaN;
  var infinity = Infinity;
  function $0() {
-  while (1) continue;
+  wasm2js_trap();
  }
  
  function $1() {