ARM64: add 128-bit MLA operations to the custom selection code.

Without this change, the llvm_unreachable kicked in. The code pattern
being spotted is rather non-canonical for 128-bit MLAs, but it can
happen and there's no point in generating sub-optimal code for it just
because it looks odd.

Should fix PR19332.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205615 91177308-0d34-0410-b5e6-96231b3b80d8
ARM64: use regalloc-friendly COPY_TO_REGCLASS for bitcasts

The previous patterns directly inserted FMOV or INS instructions into
the DAG for scalar_to_vector & bitconvert patterns. This is horribly
inefficient and can generated lots more GPR <-> FPR register traffic
than necessary.

It's much better to emit instructions the register allocator
understands so it can coalesce the copies when appropriate.

It led to at least one ISelLowering hack to avoid the problems, which
was incorrect for v1i64 (FPR64 has no dsub). It can now be removed
entirely.

This should also fix PR19331.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205616 91177308-0d34-0410-b5e6-96231b3b80d8