[ValueTracking][InstCombine] Introduce and use ComputeMinSignedBits

This introduces a new ComputeMinSignedBits method for ValueTracking that
returns the BitWidth - SignBits + 1 from ComputeSignBits, and represents
the minimum bit size for the value as a signed integer.  Similar to the
existing APInt::getMinSignedBits method, this can make some of the
reasoning around ComputeSignBits more natural.

See https://reviews.llvm.org/D112298