This CL prepares the way for adding a root register on ia32. The new
register allocation configuration PreserveRootIA32 treats
kRootRegister as an unallocatable register.

Note that kRootRegister (on ia32) is still completely unused,
unallocated, and may be clobbered at many points. This is left to
future work.

Bug: v8:6666
Change-Id: I4aacdf9c3bb365d6ed49fea8f013f79b7b1f0a98
Reviewed-on: https://chromium-review.googlesource.com/1181023Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55224}