PR #121118 attempted to introduce `checkRegister` used by
XtensaDisassembler. Since `checkRegister` and other functions in
XtensaUtils.cpp cannot link against XtensaCodeGen, move them to
XtensaDesc, which can be used by XtensaDisassembler.
Pull Request: https://github.com/llvm/llvm-project/pull/123969