[DAG] Handle build_vector with all undefs in reduceBuildVecTruncToBitCast

While working on D143731 I hit a case where a build_vector with 2 undef operands could be generated (with one undef hidden behind a bitcast).
That made `reduceBuildVecTruncToBitCast` crash because it seems to assume there is at least one good operand.

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D143886