[X86][SSE] Fix targetShrinkDemandedConstant constant vector sign extensions

D82257/rG3521ecf1f8a3 was incorrectly sign-extending a constant vector from the lsb, this is fine if all the constant elements are 'allsignbits' in the active bits, but if only some of the elements are, then we are corrupting the constant values for those elements.

This fix ensures we sign extend from the msb of the active/demanded bits instead.