The DecompressionOptimizer aims to avoid adding the root in AnyTagged
or TaggedPointer loads. For the TaggedSigned case, we already solve it
in instruction selection.

The new phase will run only when pointer compression is enabled. For
the moment, it's also requires FLAG_turbo_decompression_elimination to
be false. This latter flag is only temporary to test out the
implementation.

The phase needs to be run when Machine are present in the graph, i.e
at the very end of the pipeline. Also, since this phase may change
the load's MachineRepresentation from Tagged to Compressed, it's best
to run it as late as possible in order to keep the phases that know
about Compressed MachineRepresentation to a minimum.

As an example, if we Load a Tagged value only to Store it back again
(i.e Load -> Store nodes, with the Load being the Store's value) we
don't need to fully decompress it since the Store will ignore the
top bits.

Bug: v8:7703
Change-Id: I6b4aec203ab8cbb540b2513cabb1e2a5691ce938
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1859615
Commit-Queue: Santiago Aboy Solanes <solanes@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64358}