Fix memory leaks in TestArm64InstEmulation

Summary: We never delete the created instances, so those test fail with the memory sanitizer.

Reviewers: jasonmolenda

Reviewed By: jasonmolenda

Subscribers: aemerson, javed.absar, kristof.beyls, lldb-commits

Differential Revision: https://reviews.llvm.org/D42336

llvm-svn: 323076