AArch64/ARM64: port more tests
AArch64/ARM64: implement BFI optimisation

ARM64 was not producing pure BFI instructions for bitfield insertion
operations, unlike AArch64. The approach had to be a little different (in
ISelDAGToDAG rather than ISelLowering), and the outcomes aren't identical but
hopefully this gives it similar power.

This should address PR19424.
AArch64/ARM64: port bitfield test to ARM64.
AArch64/ARM64: enable some MC tests on ARM64

This will also (as with CodeGen) disable testing when the ARM64 backend is not
present.