[BOLT] Refactor MCInst target symbol lookup. NFCI

In analyzeInstructionForFuncReference(), use MCPlusBuilder interface
while scanning symbolic operands of MCInst. Should be NFC on x86, but
will make the function work on other architectures. Note that it's
currently unused on non-x86 as its functionality is exclusive to safe
ICF that runs on x86 only.

Author: maksfb

bolt/lib/Core/BinaryFunction.cpp

@@ -1544,15 +1544,11 @@ MCSymbol *BinaryFunction::registerBranch(uint64_t Src, uint64_t Dst) {
 }
 
 void BinaryFunction::analyzeInstructionForFuncReference(const MCInst &Inst) {
-  for (const MCOperand &Op : MCPlus::primeOperands(Inst)) {
-    if (!Op.isExpr())
+  for (unsigned OpNum = 0; OpNum < MCPlus::getNumPrimeOperands(Inst); ++OpNum) {
+    const MCSymbol *Symbol = BC.MIB->getTargetSymbol(Inst, OpNum);
+    if (!Symbol)
       continue;
-    const MCExpr &Expr = *Op.getExpr();
-    if (Expr.getKind() != MCExpr::SymbolRef)
-      continue;
-    const MCSymbol &Symbol = cast<MCSymbolRefExpr>(Expr).getSymbol();
-    // Set HasAddressTaken for a function regardless of the ICF level.
-    if (BinaryFunction *BF = BC.getFunctionForSymbol(&Symbol))
+    if (BinaryFunction *BF = BC.getFunctionForSymbol(Symbol))
       BF->setHasAddressTaken(true);
   }
 }