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VE Vector Predicated (VVP) SDNodes form an intermediate layer between VE vector instructions and the initial SDNodes. We introduce 'vvp_add' with isel and tests as the first of these VVP nodes. VVP nodes have a mask and explicit vector length operand, which we will make proper use of later. Reviewed By: kaz7 Differential Revision: https://reviews.llvm.org/D91802