Otherwise preparsed variables will cause unnecessary dynamic variable
allocation, which is especially bad when we're preparsing top-level functions
with references to other global variables.

Change-Id: I2fa17dae8c1cc5264a26ddc8b8868de1d791b0ac
Reviewed-on: https://chromium-review.googlesource.com/c/1456040
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59420}