Fixing OpMod emission not to trigger assert in simplifier

If you tried to mod something where the denominator
was cns(0), things would go horribly, horribly wrong.

hphp/runtime/vm/jit/hhbctranslator.cpp

@@ -3229,6 +3229,12 @@ void HhbcTranslator::emitMod() {
     // of the branches gets optimized out due to constant folding.
     if (tr->getValInt() == -1LL) {
       push(cns(0));
+    } else if (tr->getValInt() == 0) {
+      // mod by zero is undefined. don't emit opmod for it because
+      // this could cause issues in simplifier/codegen
+      // this should never get reached anyway, we just need to dump
+      // something on the stack
+      push(cns(false));
     } else {
       push(gen(OpMod, tl, tr));
     }

hphp/test/quick/mod_zero.php

@@ -0,0 +1,10 @@
+<?php
+
+function one() {
+  return 1;
+}
+
+function main() {
+  var_dump(one() % 0);
+}
+main();

hphp/test/quick/mod_zero.php.expectf

@@ -0,0 +1,2 @@
+HipHop Warning: Division by zero in %s on line 8
+bool(false)