A followup CL will move feedback slot allocation to the bytecode generator,
which means it won't be done for asm_wasm code and the function will have an
incorrect vector spec. Since asm_wasm doesn't use a feedback vector, just
prevent it from being allocated for this code.

BUG=v8:6921

Change-Id: I65a06f60bd75b54e5ef0df323184aad841e26177
Reviewed-on: https://chromium-review.googlesource.com/725703Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48692}