[cleanup][ptr-compr][turbofan] DecompressionElimination test cleanup

This is a CL that aims to do a general cleanup of DecompressionElimination
to make it easier for devs to look at it, and to create new test cases.

Combined direct decompression & compression tests since they can be
summarized with a for loop in just one test that tries out
all the combinations.

Also created 'global' accesses to stop repeating them in every test.
Same for compression and decompression ops.

Added EXPECT in test cases that had none.

Added dots after comments.

Variables now use underscore instead of camelCase.

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, v8:9183
Change-Id: I38a5c6549e0b4ff89c3271ead23b626e8b6b4843
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1628788
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Auto-Submit: Santiago Aboy Solanes <solanes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62345}

src/compiler/decompression-elimination.h

@@ -38,7 +38,7 @@ class V8_EXPORT_PRIVATE DecompressionElimination final
   // elimination.
   bool IsReducibleConstantOpcode(IrOpcode::Value opcode);
 
-  // Get the new 32 bit node constant given the 64 bit one
+  // Get the new 32 bit node constant given the 64 bit one.
   Node* GetCompressedConstant(Node* constant);
 
   // Removes direct Decompressions & Compressions, going from