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

[Florian Hahn via llvm-commits]

https://github.com/fhahn commented:

It looks like this is not NFC, seeing a crash for the IR below

```
target datalayout = "e-m:o-p270:32:32-p271:32:32-p272:64:64-i64:64-i128:128-n32:64-S128-Fn32"
target triple = "arm64-apple-macosx15.0.0"

define ptr @old_main() {
entry:
  br label %while.body

while.body:                                       ; preds = %while.body, %entry
  %tnow.0279 = phi double [ 0.000000e+00, %entry ], [ %add168, %while.body ]
  %add168 = fadd double %tnow.0279, 0.000000e+00
  %cmp164 = fcmp olt double %add168, 0.000000e+00
  %0 = select i1 %cmp164, i1 false, i1 false
  br label %while.body
}
```

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