[X86] combineVectorShiftImm - fold (shift (logic X, C2), C1) -> (logic (shift...

[X86] combineVectorShiftImm - fold (shift (logic X, C2), C1) -> (logic (shift X, C1), (shift C2, C1))

Helps expose a number of cases that we could/should reassociate - if only the vector constants hadn't already been lowered :(

It also forms a number of additional VPTERNLOG nodes, bringing together logic ops that had been stuck on either side of a shift op