[llvm] [KnownFPClass] Improve KnownFPClass::toKnownBits deductions for IEEE types (PR #223204)

[Eric Ross via llvm-commits]

ZERICO2005 wrote:

It is currently impossible to determine any bits for `fcNormal`. In the future, `fcNormal` may be split into a few additional flags that would allow us to determine some additional bits:
- `kfcNormalSmallerOne` (`|x| < 1.0`) MSB of exponent is clear for IEEE types.
- `kfcIntegerLargerOne` (`|x| >= 2.0` and an integer) MSB of exponent is set for IEEE types.

https://github.com/llvm/llvm-project/pull/223204