[IndVarSimplify] Refactor handleFloatingPointIV, modernize pass (NFC)

`handleFloatingPointIV` is now abstracted out into different routines,
particularly:
- `maybeFloatingPointRecurrence` which establishes whether we handle a
  floating-point iv recurrence;
- `tryConvertToIntegerIV` which attempts to convert the fp start, step
  and exit values into integer ones;
- `canonicalizeToIntegerIV` which rewrites the recurrence.

Minor opportunity to modernize the code where possible.

Author: antoniofrighetto