This is a reland of cee362af.

Original change's description:
> PPC/s390: [turbofan] Masking/poisoning in codegen (optimized code, x64)
> 
> Port 8f489e73
> 
> Original Commit Message:
> 
>     This introduces masking of loads with speculation bit during code generation.
>     At the moment, this is done only for x64 optimized code, under the
>     --branch-load-poisoning flag.
> 
>     Overview of changes:
>     - new register configuration configuration with one register reserved for
>       the speculation poison/mask (kSpeculationPoisonRegister).
>     - in codegen, we introduce an update to the poison register at the starts
>       of all successors of branches (and deopts) that are marked as safety
>       branches (deopts).
>     - in memory optimizer, we lower all field and element loads to PoisonedLoads.
>     - poisoned loads are then masked in codegen with the poison register.
>       * only integer loads are masked at the moment.
> 
> R=mvstanton@chromium.org, joransiu@ca.ibm.com, michael_dawson@ca.ibm.com
> BUG=
> LOG=N
> 
> Change-Id: I7decc16bbadf87a8c8b178278eb79a9b783f79e1
> Reviewed-on: https://chromium-review.googlesource.com/916744
> Reviewed-by: Joran Siu <joransiu@ca.ibm.com>
> Commit-Queue: Junliang Yan <jyan@ca.ibm.com>
> Cr-Commit-Position: refs/heads/master@{#51275}

Change-Id: Id22416487b05bef06c4cfdae35811a22f21cd0a0
Reviewed-on: https://chromium-review.googlesource.com/916865Reviewed-by: Joran Siu <joransiu@ca.ibm.com>
Commit-Queue: Junliang Yan <jyan@ca.ibm.com>
Cr-Commit-Position: refs/heads/master@{#51278}