[InstCombine] Teach SimplifyDemandedBits that udiv doesn't demand low dividend...

[InstCombine] Teach SimplifyDemandedBits that udiv doesn't demand low dividend bits that are zero in the divisor

This is safe as long as the udiv is not exact. The pattern is not common in
C++ code, but comes up all the time in code generated by XLA's GPU backend.

Differential Revision: https://reviews.llvm.org/D46647

llvm-svn: 331933