[BOLT][NFC] Define getExprValue helper (#91663)

Move out common code extracting the address of a MCExpr. To be reused in
#91667.

Test Plan: NFC

Author: aaupov

bolt/lib/Core/BinaryFunction.cpp

@@ -851,15 +851,19 @@ BinaryFunction::processIndirectBranch(MCInst &Instruction, unsigned Size,
     return IndirectBranchType::UNKNOWN;
   }
 
-  // RIP-relative addressing should be converted to symbol form by now
-  // in processed instructions (but not in jump).
-  if (DispExpr) {
+  auto getExprValue = [&](const MCExpr *Expr) {
     const MCSymbol *TargetSym;
     uint64_t TargetOffset;
-    std::tie(TargetSym, TargetOffset) = BC.MIB->getTargetSymbolInfo(DispExpr);
+    std::tie(TargetSym, TargetOffset) = BC.MIB->getTargetSymbolInfo(Expr);
     ErrorOr<uint64_t> SymValueOrError = BC.getSymbolValue(*TargetSym);
-    assert(SymValueOrError && "global symbol needs a value");
-    ArrayStart = *SymValueOrError + TargetOffset;
+    assert(SymValueOrError && "Global symbol needs a value");
+    return *SymValueOrError + TargetOffset;
+  };
+
+  // RIP-relative addressing should be converted to symbol form by now
+  // in processed instructions (but not in jump).
+  if (DispExpr) {
+    ArrayStart = getExprValue(DispExpr);
     BaseRegNum = BC.MIB->getNoRegister();
     if (BC.isAArch64()) {
       ArrayStart &= ~0xFFFULL;