In SimplifyDemandedUseBits, use computeKnownBits directly to handle Constants

Currently we don't explicitly process ConstantDataSequential, ConstantAggregateZero, or ConstantVector, or Undef before applying the Depth limit. Instead they occur after the depth check in the non-instruction path.

For the constant types that we do handle, the code is replicated from computeKnownBits.

This patch fixes the missing constant handling and the reduces the amount of code by just using computeKnownBits directly for any type of Constant.

Differential Revision: https://reviews.llvm.org/D32123

llvm-svn: 300849