[asmjs][cleanup] DCHECK that experimental opcodes are not used

The wasm code for asm.js modules is generated by us and does not come
from the user. Therefore we do not need to check in release builds that
experimental opcodes are not used for asm.js, a DCHECK is sufficient.

R=clemensh@chromium.org

Change-Id: I0c7135bfb03eafd2a39e648d57eff8e3a4440c3f
Reviewed-on: https://chromium-review.googlesource.com/659938Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47950}

src/wasm/function-body-decoder-impl.h

@@ -40,9 +40,7 @@ struct WasmException;
   }())
 
 #define CHECK_PROTOTYPE_OPCODE(flag)                                           \
-  if (this->module_ != nullptr && this->module_->is_asm_js()) {                \
-    this->error("Opcode not supported for asmjs modules");                     \
-  }                                                                            \
+  DCHECK(!this->module_ || !this->module_->is_asm_js());                       \
   if (!FLAG_experimental_wasm_##flag) {                                        \
     this->error("Invalid opcode (enable with --experimental-wasm-" #flag ")"); \
     break;                                                                     \