[ValueTracking] use KnownBits to compute fpclass from bitcast (#97762)

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 or rule out some fpclasses such as NaN, Zero, or Inf.

Author: AlexMaclean