[llvm] [IndVarSimplify] Refactor `handleFloatingPointIV`, modernize pass (NFC) (PR #169706)

[Alex Bradbury via llvm-commits]

asb wrote:

I'm seeing crashes on RISC-V as well and was just checking here before looking at making a small reproducer, so thanks @fhahn for beating me to it!

f54c6b4306a3f92c08aeb8a9fa222b88985cb9ef seems to have made my bot green though. Thanks @antoniofrighetto for such a rapid fix.

https://github.com/llvm/llvm-project/pull/169706