remove a double union splitting situation

[lxvm]

This could fix a type instability I noticed while working on SciML/SciMLExpectations.jl#148.
I don't have time to add tests for this.

Checklist

• Appropriate tests were added
• Any code changes were done in a way that does not break public API
• All documentation related to code changes were updated
• The new code follows the
  contributor guidelines, in particular the SciML Style Guide and
  COLPRAC.
• Any new documentation only uses public API

Additional context

Add any other context about the problem here.

[ChrisRackauckas]

This makes no difference in the SSA IR since the second assignment is a separate variable.

[lxvm]

I found out both u0 and p were inferred as unions of nothing and their intended value by using Cthulhu and @code_warntype also showed a type instability, so this caused some @inferred tests to fail. The problem is that one union gets split, but when there are two the typocalypse begins and inference gives up. I've seen it happen in lots of code. Are you saying it gets optimized away at runtime?

[lxvm]

I guess that's why they call Cthulhu the slow descent into madness 😅