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

[Cullen Rhodes via llvm-commits]

c-rhodes 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? 

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