[X86] lowerBuildVectorAsBroadcast - broadcast Constant of original (BuildVector) element size

Noticed in D150143/D150526 - we currently create scalar Constant values using the broadcast instruction width, which might be wider than the original build vector width, making it tricky to recognise the original constant bits data.

If we have widened the broadcast value, its much more useful for asm comments if we create a ConstantVector with the original element data, add that to the constant-pool and load that with the same (wider) broadcast instruction.