Improve 256-bit shuffle splitting to allow 2 sources in each 128-bit lane. As...

Improve 256-bit shuffle splitting to allow 2 sources in each 128-bit lane. As long as only a single lane of the source is used in the lane in the destination. This makes the splitting match much closer to what happens with 256-bit shuffles when AVX is disabled and only 128-bit XMM is allowed.

llvm-svn: 148537