Turbofan applies the following optimization to external reference
loads on arm64 and x64: if the root-relative offset to an external
reference's address is known to be constant (and the root register has
been initialized), calculate the external reference as |kRootRegister
+ <offset>| instead of loading it from the external reference table.

There are two main cases to consider:

1. External references to arbitrary addresses in the native address
space, e.g. libc_memcpy. These kinds of external references have a
fixed address within the same running process, but may (and likely
will) change between processes (e.g.: mksnapshot and later chromium),
and the root-relative offset is different for each Isolate within the
same process.

These kinds of external references can be optimized as above when
*not* generating code which will later be serialized, and *not*
generating isolate-independent code.

2. External references to addresses within the fixed-size region of
the Isolate (essentially: within IsolateData). Since these move with
the Isolate, their root-relative offset is guaranteed to be constant
at all times.

The optimization can always be applied to these cases as long as the
root register has been initialized.

Prior to this CL, we only recognized and optimized for case 1. This CL
additionally adds support for 2.

An example of improved code generated due to this CL:

Before:
// r13 is the kRootRegister on x64.
// 0x3010 is the root-relative offset to Isolate::context_address.
leaq rdx, [r13+0x3010]
movq r8, [rdx]

After:
movq rdx, [r13+0x3010]

Bug: v8:9534
Change-Id: Idfcca751e98a56c0e5ead2c701c12a677df75399
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1748727
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Auto-Submit: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63158}