[X86][SSE] Avoid duplicate shuffle input sources in combineX86ShufflesRecursively

[X86][SSE] Avoid duplicate shuffle input sources in combineX86ShufflesRecursively

rL339686 added the case where a faux shuffle might have repeated shuffle inputs coming from either side of the OR().

This patch improves the insertion of the inputs into the source ops lists to account for this, as well as making it trivial to add support for shuffles with more than 2 inputs in the future.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@339696 91177308-0d34-0410-b5e6-96231b3b80d8