[asm] Store source position for all loops

Loop headers contain a stack check in wasm, hence an exception can be
thrown at the position of the loop instruction. This means that for
asm.js, we need to store a source position for each loop instruction.

R=mstarzinger@chromium.org

Bug: chromium:799690
Change-Id: I129abef11461992e2f10af8e6afc28ce1cf83341
Reviewed-on: https://chromium-review.googlesource.com/856338Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50443}

src/asmjs/asm-parser.cc

@@ -292,6 +292,9 @@ void AsmJsParser::Begin(AsmJsScanner::token_t label) {
 
 void AsmJsParser::Loop(AsmJsScanner::token_t label) {
   BareBegin(BlockKind::kLoop, label);
+  int position = static_cast<int>(scanner_.Position());
+  DCHECK_EQ(position, scanner_.Position());
+  current_function_builder_->AddAsmWasmOffset(position, position);
   current_function_builder_->EmitWithU8(kExprLoop, kLocalVoid);
 }
 

test/mjsunit/regress/regress-799690.js

+// Copyright 2018 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Flags: --stack-size=500
+
+function asm() {
+  "use asm";
+  function f(a) {
+    a = a | 0;
+    while (1) return 1;
+    return 0;
+  }
+  return { f: f};
+}
+const mod = asm();
+function call_f() {
+  mod.f();
+  call_f();
+}
+assertThrows(call_f, RangeError);