[llvm] [RISCV] Expand scalar FP undef/poison to 0.0 with F extension. (PR #225567)

[John Regehr via llvm-commits]

https://github.com/regehr approved this pull request.

this is much better!

I have a couple of test cases -- just slightly more elaborate versions of the "return float poison" in my original test cases -- that are miscompiled under #225549 but they come up clean at this branch.

this is too complicated for me to reason about from first principles, all I can do is try stuff :(

https://github.com/llvm/llvm-project/pull/225567