Simplify make_seed and undo 7b44eb now that register shifts are safe

This needs to be tested more on 32-bit machines

As per discussion: https://github.com/JuliaLang/julia/commit/7b44ebe60f3e0a5e147f3124aa6ea813edadcfa0
Merge pull request #6023 from jperla/fixmakeseed

Simplify make_seed and undo 7b44eb now that register shifts are safe