[AArch64 NEON] Fix a bug caused by undef lane when generating VEXT.

It was commited as r199628 but reverted in r199628 as causing
regression test failed. It's because of old vervsion of patch
I used to commit. Sorry for mistake.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@199704 91177308-0d34-0410-b5e6-96231b3b80d8