[AArch64] Use DUP for BUILD_VECTOR with few different elements.

If most elements of BUILD_VECTOR are the same, with a few different
elements, it is better to use DUP  for the common elements and
INSERT_VECTOR_ELT for the different elements.

Currently this transform is guarded quite restrictively to only trigger
in clearly beneficial cases.

With D90176, the lowering for patterns originating from code like
` float32x4_t y = {a,a,a,0};` (common in 3D apps) are lowered even
better (unnecessary fmov is removed).

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D90233