[AArch64] Simplify AES*Tied pseudo expansion (NFC).

Summary:
Suggested by @t.p.northover in https://bugs.llvm.org/show_bug.cgi?id=34015.


Reviewers: javed.absar, t.p.northover, rengolin

Reviewed By: t.p.northover

Subscribers: aemerson, kristof.beyls, llvm-commits, t.p.northover

Differential Revision: https://reviews.llvm.org/D36223

llvm-svn: 309821