[KnownBits] Improve `KnownBits::rem(X, Y)` in cases where we can deduce low-bits of output

The first `cttz(Y)` bits in `X` are translated 1-1 in the output.

Alive2 Links:
    https://alive2.llvm.org/ce/z/Qc47p7
    https://alive2.llvm.org/ce/z/19ut5H

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D149421