[ValueTracking] use KnownBits to compute fpclass from bitcast

[AlexMaclean]

When we encounter a bitcast from an integer type we can use the information from KnownBits to glean some information about the fpclass:

• If the sign bit is known, we can transfer this information over.
• If the float is IEEE format and enough of the bits are known, we may be able to prove it is NaN or that it can never be NaN.