[ptr-compr] Remove unnecessary decompression in kArchStoreWithWriteBarrier

The object itself is already decompressed, and we're simply re-decompressing by
nuking the upper bits through sign extension.

Additionally this CL changes the branchless decompression sequence on x64 to be
cmov-based since that's shorter and faster. It's still slower than branchful
though, so we likely won't use it.

Change-Id: Ie6f9d38fb390b7300a236bf85d0db58d1ee959b0
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1701842Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62708}

src/codegen/x64/macro-assembler-x64.cc

@@ -322,10 +322,9 @@ void TurboAssembler::DecompressRegisterAnyTagged(Register destination,
     // masked_root = HAS_SMI_TAG(destination) ? 0 : kRootRegister;
     STATIC_ASSERT((kSmiTagSize == 1) && (kSmiTag < 32));
     Register masked_root = scratch;
-    movl(masked_root, destination);
-    andl(masked_root, Immediate(kSmiTagMask));
-    negq(masked_root);
-    andq(masked_root, kRootRegister);
+    xorq(masked_root, masked_root);
+    Condition smi = CheckSmi(destination);
+    cmovq(NegateCondition(smi), masked_root, kRootRegister);
     // Now this add operation will either leave the value unchanged if it is
     // a smi or add the isolate root if it is a heap object.
     addq(destination, masked_root);

src/compiler/backend/arm64/code-generator-arm64.cc

@@ -866,9 +866,6 @@ CodeGenerator::CodeGenResult CodeGenerator::AssembleArchInstruction(
           this, object, offset, value, mode, DetermineStubCallMode(),
           &unwinding_info_writer_);
       __ StoreTaggedField(value, MemOperand(object, offset));
-      if (COMPRESS_POINTERS_BOOL) {
-        __ DecompressTaggedPointer(object, object);
-      }
       __ CheckPageFlag(object, MemoryChunk::kPointersFromHereAreInterestingMask,
                        eq, ool->entry());
       __ Bind(ool->exit());

src/compiler/backend/x64/code-generator-x64.cc

@@ -1041,9 +1041,6 @@ CodeGenerator::CodeGenResult CodeGenerator::AssembleArchInstruction(
           OutOfLineRecordWrite(this, object, operand, value, scratch0, scratch1,
                                mode, DetermineStubCallMode());
       __ StoreTaggedField(operand, value);
-      if (COMPRESS_POINTERS_BOOL) {
-        __ DecompressTaggedPointer(object, object);
-      }
       __ CheckPageFlag(object, scratch0,
                        MemoryChunk::kPointersFromHereAreInterestingMask,
                        not_zero, ool->entry());