[AArch64] Optimize instruction selection for certain vector shuffles

This patch adds code to recognize vector shuffles which can be
represented as VDUP (splat) of a vector lane with of a different
(wider) type than the original vector lane type.

For example:
    shufflevector <4 x i16> %v, <4 x i16> undef, <4 x i32> <i32 0, i32 1, i32 0, i32 1>
is essentially:
    shufflevector <2 x i32> %v, <2 x i32> undef, <2 x i32> <i32 0, i32 0>

Such patterns are generated by the SelectionDAG machinery in some cases
(see DAGCombiner::visitBITCAST in DAGCombiner.cpp, the "Remove double
bitcasts from shuffles" part).

Reviewed By: dmgreen

Differential Revision: https://reviews.llvm.org/D86225