[AArch64TargetParser] reinstate assert for optional

[tmatheson]

Summary

D145206 changed value() to value_or(0), removing the assert that checks
that there is a minor version. Add it back explicitly.

llvm/include/llvm/TargetParser/AArch64TargetParser.h

struct ArchInfo {
   // AArch64TargetInfo::setFeatureEnabled.
   bool implies(const ArchInfo &Other) const {
     if (this->Profile != Other.Profile)
       return false; // ARMV8R
     if (this->Version.getMajor() == Other.Version.getMajor()) {
       return this->Version > Other.Version;
     }
     if (this->Version.getMajor() == 9 && Other.Version.getMajor() == 8) {
+      assert(this->Version.getMinor() && Other.Version.getMinor() &&
+             "AArch64::ArchInfo should have a minor version.");
       return this->Version.getMinor().value_or(0) + 5 >=
              Other.Version.getMinor().value_or(0);
     }
     return false;
   }
 
   // Return ArchFeature without the leading "+".
   StringRef getSubArch() const { return ArchFeature.substr(1); }