The isolate can be efficiently loaded through other means: either as a
root-relative load (if embedded builtins are enabled), or as an embedded
external reference (i.e. the absolute pointer is included in the
instruction stream) otherwise.

The generated code should be at least as fast as previously. On x64
(with embedded builtins):

Before:
 // Register moves in prologue:
 0x7f47a6b4860a     a  488955e0       REX.W movq [rbp-0x20],rdx
 // And the load from a stack slot at each use-site.
 0x7f47a6b486f2    f2  488b7de0       REX.W movq rdi,[rbp-0x20]

After:
 // Each use-site just loads a root-relative offset.
 0x7f1645fcc6ce    ee  498dbd38ffffff REX.W leaq rdi,[r13-0xc8]

On ia32 (no embedded builtins), before:
 0x5c608930    10  8955f0         mov [ebp-0x10],edx
 0x5c6089fb    db  891424         mov [esp],edx

After:
 0x41d0898d    8d  b80033b156     mov eax,0x56b13300

Removal reduces register pressure, and frees up ebx as the root register
on ia32.

Note that the set of allocatable registers was only reduced on ia32 to
exclude the root register.

Bug: v8:6666
Change-Id: I14e401e2823c82042c76acae10c3c935b9982993
Reviewed-on: https://chromium-review.googlesource.com/1201586
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55587}