(Initially copied from nicohartmann@ CL
https://chromium-review.googlesource.com/c/v8/v8/+/2135631)

This CL adds a new intrinsic %ObserveNode(expr) which has noop semantics
but triggers the new NodeObserver set on the OptimizedCompilationInfo
when the node generated for expr is created or changed in any phase
(until EffectControlLinearization).

This provides the infrastructure to write reasonable unit tests that
check for the construction of or lowering to specific nodes (e.g.
depending on feedback).

When %ObserveNode(expr) is used an object of class ObserveNodeManager is
registered to every Reducer/GraphReducer and is notified by the Reducer
with all node changes. The same logic is added to classes
SimplifiedLowering/RepresentationSelector, which do not inherit from
class Reducer.

Observed Node modifications currently are:
 * The Node Operator
 * The Node type
 * Node replacements

A first use case (cctest/test-sloppy-equality.cc) is included in this CL.

Change-Id: Idc5a5e38af8b1d9a2ec5021bf821c4e4e1406220
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2555219
Commit-Queue: Paolo Severini <paolosev@microsoft.com>
Reviewed-by: Georg Neis <neis@chromium.org>
Reviewed-by: Nico Hartmann <nicohartmann@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72331}