[SelectionDAG] Add BITCAST handling to ComputeNumSignBits for splatted sign bits.

[SelectionDAG] Add BITCAST handling to ComputeNumSignBits for splatted sign bits.

For cases where we are BITCASTing to vectors of smaller elements, then if the entire source was a splatted sign (src's NumSignBits == SrcBitWidth) we can say that the dst's NumSignBit == DstBitWidth, as we're just splitting those sign bits across multiple elements.

We could generalize this but at the moment the only use case I have is to peek through bitcasts to vector comparison results.

Differential Revision: https://reviews.llvm.org/D37849

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@313543 91177308-0d34-0410-b5e6-96231b3b80d8