Implement highbd_d63_predictor using Neon

Implement highbd_d63_predictor using Neon

Add Neon implementations of the highbd d63 predictor for 4x4, 8x8, 16x16
and 32x32 block sizes. Also update tests to add new corresponding cases.

This re-lands commit 7cdf139e3d6237386e0f93bdb0bdc1b459c663bf,
previously reverted in 7478b7e4e481562a4a13f233acb66a60462e1934.

Compared to the previous implementation attempt we now correctly match
the behaviour of the C code when handling the final element loaded from
the 'above' input array. In particular:

- The C code for a 4x4 block performs a full average of the last element
  rather than duplicating the final element from the input 'above'
  array.

- The C code for other block sizes performs a full average for the
  stride=0 and stride=1, and otherwise shifts in duplicates of the final
  element from the input 'above' array. Notably this shifting for later
  strides _replaces_ the final element which we previously performed an
  average on (see {d0,d1}_ext in the code).

It is worth noting that this difference is not caught by the existing
VP9HighbdIntraPredTest test cases since the test vector initialisation
contains this loop:

    for (int x = block_size; x < 2 * block_size; x++) {
        above_row_[x] = above_row_[block_size - 1];
    }

Since AVG2(a, a) and AVG3(a, a, a) are simply 'a', such differences in
behaviour for the final element are not observed.

Tested on AArch64 with:

- ./test_libvpx --gtest_filter="*VP9HighbdIntraPredTest*"
- ./test_libvpx --gtest_filter="*VP9/TestVectorTest.MD5Match*"
- ./test_libvpx --gtest_filter="*VP9/ExternalFrameBufferMD5Test*"

Speedups over the C code (higher is better):

Microarch.  | Compiler | Block | Speedup
Neoverse N1 |  LLVM 15 |   4x4 |    2.43
Neoverse N1 |  LLVM 15 |   8x8 |    3.92
Neoverse N1 |  LLVM 15 | 16x16 |    3.19
Neoverse N1 |  LLVM 15 | 32x32 |    4.13
Neoverse N1 |   GCC 12 |   4x4 |    2.92
Neoverse N1 |   GCC 12 |   8x8 |    6.51
Neoverse N1 |   GCC 12 | 16x16 |    4.55
Neoverse N1 |   GCC 12 | 32x32 |    3.18
Neoverse V1 |  LLVM 15 |   4x4 |    1.99
Neoverse V1 |  LLVM 15 |   8x8 |    3.65
Neoverse V1 |  LLVM 15 | 16x16 |    3.72
Neoverse V1 |  LLVM 15 | 32x32 |    3.26
Neoverse V1 |   GCC 12 |   4x4 |    2.39
Neoverse V1 |   GCC 12 |   8x8 |    4.76
Neoverse V1 |   GCC 12 | 16x16 |    3.24
Neoverse V1 |   GCC 12 | 32x32 |    2.44

Change-Id: Iefaa774d6a20388b523eaa7f5df6bc5f5cf249e4