[llvm] [SDAG] Harden assumption in getMemsetStringVal (PR #126207)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> > This sounds OK, but could it use a fp type that doesn't have a representation of 0.0, or where 0.0 != 0x0000? I doubt that would be used in practice, but is it worth having an assert to check?
> 
> any pointers on how i can check that?

We don't have any of those in the IR, if they exist. 

https://github.com/llvm/llvm-project/pull/126207