This also removes a broken optimization regarding immutable (i.e. const)
global variables. For now mutable and immutable global variables are
treated the same and hence copied during module initialization.

R=rossberg@chromium.org
TEST=mjsunit/asm/global-imports
BUG=v8:6279

Change-Id: I020fc12036dc534f5a62fb43f5c6fdb252314e62
Reviewed-on: https://chromium-review.googlesource.com/483360Reviewed-by: Andreas Rossberg <rossberg@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44763}