AArch64/ARM64: enable more AArch64 tests.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@209408 91177308-0d34-0410-b5e6-96231b3b80d8
ARM64: assert if we see i64 -> i64 extend in the DAG.

Should be no change in behaviour, but it makes the intended
functionality a bit clearer and means we only have to reason about
real extend operations.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@209409 91177308-0d34-0410-b5e6-96231b3b80d8