Revert of Fix clobbered register when setting this_function variable. (patchset #2 id:20001 of https://codereview.chromium.org/1185703002/)

Reason for revert:
[Sheriff] Makes mjsunit/es6/block-const-assign flaky, e.g.:
http://build.chromium.org/p/client.v8/builders/V8%20Linux/builds/4082

Original issue's description:
> Fix clobbered register when setting this_function variable.
>
> R=arv@chromium.org
> TEST=mjsunit/regress/regress-crbug-498022
> BUG=chromium:498022
> LOG=N
>
> Committed: https://crrev.com/bf2bbc8ba508ccd21edf3c08d2e4192c4764ae91
> Cr-Commit-Position: refs/heads/master@{#29020}

TBR=arv@chromium.org,rossberg@chromium.org,mstarzinger@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=chromium:498022

Review URL: https://codereview.chromium.org/1184093003

Cr-Commit-Position: refs/heads/master@{#29022}