[SelectionDAG] soften splat mask assert/unreachable (PR41535)

These are general queries, so they should not die when given
a degenerate input like an all undef mask. Callers should be
able to deal with an op that will eventually be simplified away.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@358761 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed