[BOLT][AArch64] Speed up ICF pass (#172783)

Add hashing support for MCSpecifierExpr, which is frequently used as an
operand in AArch64 instructions. Proper hashing reduces collisions when
placing functions into buckets, resulting in shorter Identical Code
Folding (ICF) pass runtime.

In one benchmark, the ICF wall time decreased from 272s to 124s.

bolt/lib/Core/HashUtilities.cpp

@@ -67,7 +67,11 @@ std::string hashExpr(BinaryContext &BC, const MCExpr &Expr) {
         .append(hashInteger(BinaryExpr.getOpcode()))
         .append(hashExpr(BC, *BinaryExpr.getRHS()));
   }
-  case MCExpr::Specifier:
+  case MCExpr::Specifier: {
+    const auto &SpecExpr = cast<MCSpecifierExpr>(Expr);
+    return hashInteger(SpecExpr.getSpecifier())
+        .append(hashExpr(BC, *SpecExpr.getSubExpr()));
+  }
   case MCExpr::Target:
     return std::string();
   }