Liftoff needs to emit the same code, thus move it to a place where it
can be reused.
Also, switch the _MSC_VER switch to V8_OS_WIN. It is unclear if
_MSC_VER would also be set in clang. V8_OS_WIN seems to fit better.

Drive-by: Reorder the code to save one jmp in the loop (loop inversion).

R=mstarzinger@chromium.org

Bug: v8:7908, v8:6600
Change-Id: Ia76aa4dad3f96e179a9daad8c1551b1aab499878
Reviewed-on: https://chromium-review.googlesource.com/1120174
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54113}