[InstSimplify] Add more poison folding optimizations

This adds more poison folding optimizations to InstSimplify.

Since all binary operators propagate poison, these are fine.

Also, the precondition of `select cond, undef, x` -> `x` is relaxed to allow the case when `x` is undef.

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D104661