[AArch64] Asm: Fix parsing of register aliases that have a name starting with 'z'

Summary: This fixes an issue as identified by @rnk in https://reviews.llvm.org/rL321029.

Reviewers: rnk, fhahn, rengolin, efriedma, echristo, olista01

Reviewed By: rnk, fhahn

Subscribers: aemerson, javed.absar, kristof.beyls, llvm-commits, rnk

Differential Revision: https://reviews.llvm.org/D41382

llvm-svn: 321158