[AArch64AsmParser] Fix type-limits warning for VectorIndex.

Making VectorIndex an `int` instead of `unsigned`, silences the warning:
  comparison of unsigned expression in ‘>= 0’ is always true

in:
  template <int Min, int Max>
  DiagnosticPredicate isVectorIndex() const {
    ...
    if (VectorIndex.Val >= Min && VectorIndex.Val <= Max)
      return DiagnosticPredicateTy::Match;
    ...
  }

when Min is 0.