[AArch64] Fix SelectionDAG infinite loop for v1i64 SCALAR_TO_VECTOR

A consequence of r347274 is that SCALAR_TO_VECTOR can be converted into

BUILD_VECTOR by SimplifyDemandedBits, but LowerBUILD_VECTOR can turn

BUILD_VECTOR into SCALAR_TO_VECTOR so we get an infinite loop.

Fix this by making LowerBUILD_VECTOR not do this transformation for those

vectors that would get transformed back, i.e. BUILD_VECTOR of a single-element

constant vector. Doing that means we get a DUP, which we then need to recognise

in ISel as a copy.