[llvm] [KnownFPClass] Refine known classes for `KnownFPClass::bitcast` (PR #215708)

[via llvm-commits]

ZERICO2005 wrote:

Renamed `nofpclass-bitcast.ll` to `nofpclass-bitcast-fp-to-fp.ll`, and removed the unused TUNIT code for that file.

The new `lit` tests are now inside of `nofpclass-bitcast-int-to-fp.ll`.

Moved `bitcast` tests out of `nofpclass.ll`, and into `nofpclass-bitcast-int-to-fp.ll`.
Renamed tests:
```
bitcast_to_float_sign_1 --> bitcast_to_float_negative_sign
bitcast_to_float_sign_0 --> bitcast_to_float_lshr_1
bitcast_to_float_nnan --> bitcast_to_float_lshr_2
bitcast_to_double_sign_0 --> bitcast_to_double_lshr_1
bitcast_to_double_nnan --> bitcast_to_double_lshr_2
```

https://github.com/llvm/llvm-project/pull/215708