Changes the handling of TestResultScopes to allow them to be reused by
logical tests by rewiring instead of using a new TestResultScope. 
Also does the following:
 - moves some fields about in TestResultScope to reduce it's size
 - moves RegisterListFreeEvent to the end of ReleaseRegisters to enable
   it to be tail-called.

This increases the allowed depth of logical expressions which the
compiler can handle without overflowing the stack by about 2x on x64.

BUG=chromium:731861

Change-Id: I7733797bec5e52d07eec6332c07e2a886f2bbde1
Reviewed-on: https://chromium-review.googlesource.com/539521
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46039}