[BOLT][NFC] Be more obvious about selecting X86 (#88527)

Use `isX86()` rather than `!isAArch64() && !isRISCV()`, and similar.

Author: urnathan

bolt/lib/Core/BinaryContext.cpp

@@ -1880,7 +1880,7 @@ MarkerSymType BinaryContext::getMarkerType(const SymbolRef &Symbol) const {
   // For aarch64 and riscv, the ABI defines mapping symbols so we identify data
   // in the code section (see IHI0056B). $x identifies a symbol starting code or
   // the end of a data chunk inside code, $d identifies start of data.
-  if ((!isAArch64() && !isRISCV()) || ELFSymbolRef(Symbol).getSize())
+  if (isX86() || ELFSymbolRef(Symbol).getSize())
     return MarkerSymType::NONE;
 
   Expected<StringRef> NameOrError = Symbol.getName();

bolt/lib/Core/BinaryEmitter.cpp

@@ -512,7 +512,7 @@ void BinaryEmitter::emitFunctionBody(BinaryFunction &BF, FunctionFragment &FF,
 
       // Emit sized NOPs via MCAsmBackend::writeNopData() interface on x86.
       // This is a workaround for invalid NOPs handling by asm/disasm layer.
-      if (BC.MIB->isNoop(Instr) && BC.isX86()) {
+      if (BC.isX86() && BC.MIB->isNoop(Instr)) {
         if (std::optional<uint32_t> Size = BC.MIB->getSize(Instr)) {
           SmallString<15> Code;
           raw_svector_ostream VecOS(Code);

bolt/lib/Rewrite/RewriteInstance.cpp

@@ -1670,7 +1670,9 @@ void RewriteInstance::disassemblePLT() {
       return disassemblePLTSectionAArch64(Section);
     if (BC->isRISCV())
       return disassemblePLTSectionRISCV(Section);
-    return disassemblePLTSectionX86(Section, EntrySize);
+    if (BC->isX86())
+      return disassemblePLTSectionX86(Section, EntrySize);
+    llvm_unreachable("Unmplemented PLT");
   };
 
   for (BinarySection &Section : BC->allocatableSections()) {
@@ -2605,7 +2607,7 @@ void RewriteInstance::handleRelocation(const SectionRef &RelocatedSection,
   const bool IsToCode = ReferencedSection && ReferencedSection->isText();
 
   // Special handling of PC-relative relocations.
-  if (!IsAArch64 && !BC->isRISCV() && Relocation::isPCRelative(RType)) {
+  if (BC->isX86() && Relocation::isPCRelative(RType)) {
     if (!IsFromCode && IsToCode) {
       // PC-relative relocations from data to code are tricky since the
       // original information is typically lost after linking, even with