AArch64: make register block rules apply to vector types too.

The blocking code originated in ARM, which is more aggressive about casting
types to a canonical representative before doing anything else, so I missed out
most vector HFAs and broke the ABI. This should fix it.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223126 91177308-0d34-0410-b5e6-96231b3b80d8