[AArch64] Skip isZIPMask check for masks with an odd number of elements.

We process 2 elements at a time and expect the number of elements to be
even. Similar to D60690.

Reviewers: dmgreen, samparker, t.p.northover

Reviewed By: dmgreen

Differential Revision: https://reviews.llvm.org/D65400

llvm-svn: 367831