Used by ReduceJSEqualTypeOf, and will also be used to lower a new 
TestTypeof bytecode in a followup CL.

BUG=v8:5267

Change-Id: I990aa6ac8ac0b9bd01080dda1764c5bfe3a4d7cf
Reviewed-on: https://chromium-review.googlesource.com/454797Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#43802}