[Float2Int] Fix pessimization in the MinBW calculation. (#86051)

The MinBW was being calculated using the significant bits of the upper
and lower bounds. The upper bound is 1 past the last value in the range
so I don't think it should be included. Instead use ConstantRange::getMinSignedBits.

I'm still not sure if the +1 is needed after the getMinSignedBits call.