[X86][SSE] Enable target shuffle combining to combine multiple shuffle inputs.

We currently only support combining target shuffles that consist of a single source input (plus elements known to be undef/zero).

This patch generalizes the recursive combining of the target shuffle to collect all the inputs, merging any duplicates along the way, into a full set of src ops and its shuffle mask.

We uncover a number of cases where we have failed to combine a unary shuffle because the input has been duplicated and separated during lowering.

This will allow us to combine to 2-input shuffles in a future patch.

Differential Revision: https://reviews.llvm.org/D22859

llvm-svn: 277631