[X86] combinePredicateReduction - ensure non-i8 scalar type is a multiple of 8 bits

For the "all_of(setcc(x,y,eq)) -> PMOVMSKB(PCMPEQB())" fold, we failed to ensure that we could safely bitcast to <X x i8>, which in particular failed with boolean types

Thanks to @lerno for catching this and providing the test case