[InstCombine] Provide a way to calculate KnownZero/One for Add/Sub in...

[InstCombine] Provide a way to calculate KnownZero/One for Add/Sub in SimplifyDemandedUseBits without recursing into ComputeKnownBits

SimplifyDemandedUseBits for Add/Sub already recursed down LHS and RHS for simplifying bits. If that didn't provide any simplifications we fall back to calling computeKnownBits which will recurse again. Instead just take the known bits for LHS and RHS we already have and call into a new function in ValueTracking that can calculate the known bits given the LHS/RHS bits.

llvm-svn: 298711