handle dangerous sub-ifs aside from the final child, when deciding which if braces can be removed

tests/optimizer/asmLastOpts-output.js

@@ -6,5 +6,18 @@ function test() {
  if (HEAP32[i5 >> 2] | 0) {
   if ((_fread(i4 | 0, 1, 257, HEAP32[i5 >> 2] | 0) | 0) != 257) if ((HEAPU8[i4 >> 0] | 0 | 0) != (HEAP32[84328] | 0)) if (_memcmp(i4 + 1 | 0, HEAP32[i20 >> 2] | 0, 256) | 0) if ((_fread(HEAP32[i16 >> 2] | 0, 256, 256, HEAP32[i5 >> 2] | 0) | 0) != 256) i31 = 9;
  } else i31 = 9;
+ moar();
+ if ((c[21902] | 0) != 0 & (Vb | 0) == 29) {
+  if ((Ib | 0) == 60) if (!(cq(39080, 448, c[F >> 2] | 0, c[M >> 2] | 0, 0, 0) | 0)) x = 373; else {
+   Mb = 52;
+   Nb = 0;
+   Ob = 0;
+   return;
+  }
+ } else x = 373;
+ yet();
+ if (a) if (b) {
+  if (c) d();
+ } else e(); else f();
 }
 

tests/optimizer/asmLastOpts.js

@@ -32,6 +32,31 @@ function test() {
  } else {
   i31 = 9;
  }
+ moar();
+ if ((c[21902] | 0) != 0 & (Vb | 0) == 29) {
+  if ((Ib | 0) == 60) {
+   if (!(cq(39080, 448, c[F >> 2] | 0, c[M >> 2] | 0, 0, 0) | 0)) {
+    x = 373;
+   } else {
+    Mb = 52;
+    Nb = 0;
+    Ob = 0;
+    return;
+   }
+  }
+ } else x = 373;
+ yet();
+ if (a) {
+  if (b) {
+   if (c) {
+    d();
+   }
+  } else {
+   e();
+  }
+ } else {
+  f();
+ }
 }
 // EMSCRIPTEN_GENERATED_FUNCTIONS: ["test"]
 

tools/optimizer/simple_ast.h

@@ -1121,21 +1121,18 @@ struct JSPrinter {
     // also need to recurse for                                if () { if () { } else { if () } else
     // (note that this is only a problem if the if body has a single element in it, not a block or such, as then
     // the block would be braced)
+    // this analysis is a little conservative - it assumes any child if could be confused with us, which implies
+    // all other braces vanished (the worst case for us, we are not saved by other braces).
     bool needBraces = false;
     bool hasElse = ifHasElse(node);
     if (hasElse) {
       Ref child = node[2];
       while (child[0] == IF) {
-        Ref last = ifHasElse(child) ? child[3] : child[2];
-        if (last[0] == IF) {
-          child = last;
-          continue;
-        }
-        // we are at the top, the one dangerous to be confused with us
         if (!ifHasElse(child)) {
           needBraces = true;
+          break;
         }
-        break;
+        child = child[3]; // continue into the else
       }
     }
     if (needBraces) {