[InstCombine] Fix known bits handling in SimplifyDemandedUseBits

Fixes a regression from D75801. SimplifyDemandedUseBits() is also
supposed to compute the known bits (of the demanded subset) of the
instruction. For unknown instructions it does so by directly calling
computeKnownBits(). For known instructions it will compute known
bits itself. However, for instructions where only some cases are
handled directly (e.g. a constant shift amount) the known bits
invocation for the unhandled case is sometimes missing. This patch
adds the missing calls and thus removes the main discrepancy with
ExpensiveCombines mode.

Differential Revision: https://reviews.llvm.org/D75804