AArch64/ARM64: improve spotting of EXT instructions from VECTOR_SHUFFLE.

We couldn't cope if the first mask element was UNDEF before, which
isn't ideal.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@206588 91177308-0d34-0410-b5e6-96231b3b80d8