[SelectionDAG] Don't pass on DemandedElts when handling SCALAR_TO_VECTOR

[SelectionDAG] Don't pass on DemandedElts when handling SCALAR_TO_VECTOR

Fixes an assertion:

llc: lib/CodeGen/SelectionDAG/SelectionDAG.cpp:2200: llvm::KnownBits llvm::SelectionDAG::computeKnownBits(llvm::SDValue, const llvm::APInt&, unsigned int) const: Assertion `(!Op.getValueType().isVector() || NumElts == Op.getValueType().getVectorNumElements()) && "Unexpected vector size"' failed.

Committed on behalf of: @pendingchaos (Rhys Perry)

Differential Revision: https://reviews.llvm.org/D55223

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@348574 91177308-0d34-0410-b5e6-96231b3b80d8