[AArch64] Skip mask checks for masks with an odd number of elements.

Some checks in isShuffleMaskLegal expect an even number of elements,
e.g. isTRN_v_undef_Mask or isUZP_v_undef_Mask, otherwise they access
invalid elements and crash. This patch adds checks to the impacted
functions.

Fixes PR41951

Reviewers: t.p.northover, dmgreen, samparker

Reviewed By: dmgreen

Differential Revision: https://reviews.llvm.org/D60690

llvm-svn: 361235