- Handle special scalar_to_vector case: splats. Using a native 128-bit

- Handle special scalar_to_vector case: splats. Using a native 128-bit
shuffle before inserting on a 256-bit vector.
- Add AVX versions of movd/movq instructions
- Introduce a few COPY patterns to match insert_subvector instructions.
This turns a trivial insert_subvector instruction into a register copy,
coalescing the xmm into a ymm and avoid emiting on more instruction.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@136002 91177308-0d34-0410-b5e6-96231b3b80d8