This reverts two commits:

Introduce CodeAssembler::LoadRootsPointer
377803f8

[turbofan][x64] Reduce reg-to-reg moving instruction for
LoadRootsRegister IR
d4177d11

LoadRootsPointer was used by indirections for heap constants and
external references from within CSA. Now that handling has moved to
the macro-assembler, it can be removed.

Bug: v8:6666
Change-Id: I868fe100e65a0a7a44ffc81674fa1ce79a56f7ed
Reviewed-on: https://chromium-review.googlesource.com/1097080
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53770}