[AArch64] Unit test for zext lowering for different types of vectors

These are unit tests for code generated for zext lowering of vectors.
There are tests for different types of vectors, as well as cases where multiple 'zext' instructions of same type occur back-to-back.
Testing of Global-ISel path is added.
This will help in comparing changes in zext lowering in https://reviews.llvm.org/D136722.

Reviewed By: fhahn

Differential Revision: https://reviews.llvm.org/D137993