[ValueTracking][SelectionDAG] Rename ComputeMinSignedBits->ComputeMaxSignificantBits. NFC

This function returns an upper bound on the number of bits needed
to represent the signed value. Use "Max" to match similar functions
in KnownBits like countMaxActiveBits.

Rename APInt::getMinSignedBits->getSignificantBits. Keeping the old
name around to keep this patch size down. Will do a bulk rename as
follow up.

Rename KnownBits::countMaxSignedBits->countMaxSignificantBits.

Reviewed By: lebedev.ri, RKSimon, spatel

Differential Revision: https://reviews.llvm.org/D116522