Fix CodeGen/arm64-microsoft-status-reg.cpp test

Summary: This test is failing after r356499 (verified with `ninja check-clang-codegen`). Update the register selection used in the test from x0 to x8.

Reviewers: arsenm, MatzeB, efriedma

Reviewed By: efriedma

Subscribers: efriedma, wdng, javed.absar, kristof.beyls, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D59557

llvm-svn: 356517