For some reason the C++ compiler fails to realise that next_ cannot
change on entry into Scan from Next, and re-loads it, creating what
looks like a data dependency that stalls the next instruction.

Passing through a cached next_ value cleans up the generated code.

Change-Id: Iab62ed1890a3a720e5fa90a90e802305e3d55a82
Reviewed-on: https://chromium-review.googlesource.com/c/1331551
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57464}