PPC: Check R bit is not set with prefixed instructions

CL also adds TODO items to implement this functionality on
disassembler and the simulator.

The R bit which enables PC-relative capabilities when set is
not being used during codegen at the moment.

Change-Id: I0927d9be37a50beecf9b6a052e378dca7604513b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3584535Reviewed-by: Junliang Yan <junyan@redhat.com>
Commit-Queue: Milad Farazmand <mfarazma@redhat.com>
Cr-Commit-Position: refs/heads/main@{#79976}

src/diagnostics/ppc/disasm-ppc.cc

@@ -466,6 +466,8 @@ void Decoder::DecodeExtP(Instruction* instr) {
   switch (EXTP | (instr->BitField(25, 25))) {
     case PLOAD_STORE_8LS:
     case PLOAD_STORE_MLS: {
+      // TODO(miladfarca): Decode the R bit.
+      DCHECK_NE(instr->Bit(20), 1);
       // Read prefix.
       SetAsPrefixed(instr->Bits(17, 0));
       // Read suffix (next instruction).

src/execution/ppc/simulator-ppc.cc

@@ -1544,6 +1544,9 @@ void Simulator::ExecuteGeneric(Instruction* instr) {
       // Prefixed instructions.
     case PLOAD_STORE_8LS:
     case PLOAD_STORE_MLS: {
+      // TODO(miladfarca): Simulate PC-relative capability indicated by the R
+      // bit.
+      DCHECK_NE(instr->Bit(20), 1);
       // Read prefix value.
       uint64_t prefix_value = instr->Bits(17, 0);
       // Read suffix (next instruction).