Effects: Add missing combining logic for MayNotReturn (#6635)

Without that logic we could end up dropping that particular effect. This actually
made a test pass when it should not: the modified test here has a function with
effects that are ok to remove, but it had a loop which adds MayNotReturn which
we should actually not remove, so it was removed erroneously.

To fix the test, add other effects there (local ones) that we can see are removable.
Also add a function with a loop to test that we do not remove an infinite loop,
which adds coverage for the fix here.

Author: kripken

src/ir/effects.h

@@ -364,6 +364,7 @@ class EffectAnalyzer {
     isAtomic = isAtomic || other.isAtomic;
     throws_ = throws_ || other.throws_;
     danglingPop = danglingPop || other.danglingPop;
+    mayNotReturn = mayNotReturn || other.mayNotReturn;
     for (auto i : other.localsRead) {
       localsRead.insert(i);
     }