[AArch64] Known bits for AArch64ISD::DUP

An AArch64ISD::DUP is just a splat, where the known bits for each lane
are the same as the input. This teaches that to computeKnownBitsForTargetNode.

Problems arise for constants though, as a constant BUILD_VECTOR can be
lowered to an AArch64ISD::DUP, which SimplifyDemandedBits would then
turn back into a constant BUILD_VECTOR leading to an infinite cycle.
This has been prevented by adding a isTargetCanonicalConstantNode node
to prevent the conversion back into a BUILD_VECTOR.

Differential Revision: https://reviews.llvm.org/D128144