AArch64: refactor ReconstructShuffle function

Quite a bit of cruft had accumulated as we realised the various different cases
it had to handle and squeezed them in where possible. This refactoring mostly
flattens the logic and special-cases. The result is slightly longer, but I
think clearer.

Should be no functionality change.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213867 91177308-0d34-0410-b5e6-96231b3b80d8