The DecompressionOptimizationPhase performs MachineRepresentation
changes in the nodes, which causes the node's instruction selection
to avoid decompression.

It is safe to run after we already have a schedule since it does not
drop any nodes.

Bug: v8:9684, v8:7703
Change-Id: I636ae80fa82d0c78878756e9f39e7a14c02803b0
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2324252
Commit-Queue: Santiago Aboy Solanes <solanes@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69137}