[AArch64] Enable use of reduction intrinsics.


[AArch64] Enable use of reduction intrinsics.

The new experimental reduction intrinsics can now be used, so I'm enabling this
for AArch64. We will need this for SVE anyway, so it makes sense to do this for
NEON reductions as well.

The existing code to match shufflevector patterns are replaced with a direct
lowering of the reductions to AArch64-specific nodes. Tests updated with the
new, simpler, representation.

Differential Revision: https://reviews.llvm.org/D32247