[X86][SSE] Simplify demanded elements from BROADCAST shuffle source.

[X86][SSE] Simplify demanded elements from BROADCAST shuffle source.

If broadcasting from another shuffle, attempt to simplify it.

We can probably generalize this a lot more (embedding in combineX86ShufflesRecursively), but BROADCAST is one of the more troublesome as it accepts inputs of different sizes to the result.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@323602 91177308-0d34-0410-b5e6-96231b3b80d8