[X86][SSE] Attempt to widen MOVMSK vector input if the signbits are splatted.

As shown on PR37087, if we have a MOVMSK(BICAST(X)) from a wider vector, then by using MOVMSK from the wider type (32/64-bit elements) we can improve the chances of further combines with SimplifyDemandedBits/Elts and on some targets (skylake) can be more efficient.