[X86][SSE] Blend any v8i16/v4i32 shift with 2 shift unique values (REAPPLIED)

We were only doing this for basic blends, despite shuffle lowering now being good enough to handle more complex blends. This means that the two v8i16 splat shifts are performed in parallel instead of serially as the general shift case.

Reapplied with a fixed (extra null tests) version of rL336113 after reversion in rL336189 - extra test case added at rL336247.

llvm-svn: 336250