This reduction replaces the Phi's input decompressions with their parent
node, if and only if all of the Phi's inputs are Decompress nodes.

Also, if we have different Decompress nodes as inputs, we need to use
a conservative decompression after the Phi.

Cq-Include-Trybots: luci.v8.try:v8_linux64_pointer_compression_rel_ng
Cq-Include-Trybots: luci.v8.try:v8_linux64_arm64_pointer_compression_rel_ng
Bug: v8:8977, v8:7703
Change-Id: I8cc0264f9d08fe5ad25364f18c9f305afc54529c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1624785Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Commit-Queue: Santiago Aboy Solanes <solanes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61820}