[AArch64][AsmParser] Fix debug output in a few instructions

Summary:
In the parsing of BTIHint, PSBHint and Prefetch the identifier token
should be lexed after creating the operand, otherwise the StringRef is
moved before being copied and the debug output is incorrect.

Prefetch example:

    $ echo "prfm   pldl1keep, [x2]" | ./bin/llvm-mc \
        -triple aarch64-none-linux-gnu -show-encoding -debug

    Before:

      Matching formal operand class MCK_Prefetch against actual operand at
      index 1 (<prfop ,>): match success using generic matcher

    After:

      Matching formal operand class MCK_Prefetch against actual operand at
      index 1 (<prfop pldl1keep>): match success using generic matcher

Reviewed By: david-arm

Differential Revision: https://reviews.llvm.org/D80620