[AArch64] Use default member initialization (NFC)

Identified with modernize-use-default-member-init.

llvm/lib/Target/AArch64/AsmParser/AArch64AsmParser.cpp

@@ -127,7 +127,7 @@ private:
       return Prefix;
     }
 
-    PrefixInfo() : Active(false), Predicated(false) {}
+    PrefixInfo() = default;
     bool isActive() const { return Active; }
     bool isPredicated() const { return Predicated; }
     unsigned getElementSize() const {
@@ -141,8 +141,8 @@ private:
     }
 
   private:
-    bool Active;
-    bool Predicated;
+    bool Active = false;
+    bool Predicated = false;
     unsigned ElementSize;
     unsigned Dst;
     unsigned Pg;