[AArch64] Fix a silent codegen fault in BUILD_VECTOR lowering.

We should be talking about the number of source elements, not the number of destination elements, given we know at this point that the source and dest element numbers are not the same.

While we're at it, avoid writing to std::vector::end()...

Bug found with random testing and a lot of coffee.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220051 91177308-0d34-0410-b5e6-96231b3b80d8