[KnownBits] Merge the minimum shift amount and leading/trailing shift value bits handling.

By starting with the source shift value minimum leading/trailing bits, we can then add the minimum known shift amount to more accurately predict the minimum leading/trailing bits of the result.

This is currently only covered by the exhaustive unit tests in KnownBitsTests.cpp, but will help with some of the regressions encountered in D90479 (PR44526).