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, 6 insertions, 2 deletions

diff --git a/test/passes/remove-unused-names_remove-unused-brs_vacuum.txt b/test/passes/remove-unused-names_remove-unused-brs_vacuum.txt
index 83be83c5a..ca856396e 100644
--- a/test/passes/remove-unused-names_remove-unused-brs_vacuum.txt
+++ b/test/passes/remove-unused-names_remove-unused-brs_vacuum.txt
@@ -71,8 +71,12 @@
    (block $label$3
     (block
      (if
-      (i32.eqz
-       (local.get $var$8)
+      (local.get $var$8)
+      (loop $label$8
+       (if
+        (local.get $var$3)
+        (br $label$8)
+       )
       )
       (if
        (i32.eqz