[AArch64] Fix some failures exposed by value type v4f16 and v8f16.

1) Add some missing bitcast patterns for v8f16.
2) Add type promotion for operand of ld/st operations.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216706 91177308-0d34-0410-b5e6-96231b3b80d8