In preparation for kRootRegister support on ia32.

LoadWithVector now passes the vector on the stack.
LoadGlobalWithVector is able to reuse edx (used as ReceiverRegister in
LoadWithVector).

Drive-by: Aliasing DCHECKs and updated unused Register accessors.

Bug: v8:6666
Change-Id: If0e93d22c3c11f764bc0805c3e6a51654cdcc5f9
Reviewed-on: https://chromium-review.googlesource.com/1193891
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55490}