[ConstantFold] Fold operations to poison if possible

This patch updates ConstantFold, so operations are folded into poison if possible.

<alive2 proofs>
casts: https://alive2.llvm.org/ce/z/WSj7rw
binary operations (arithmetic): https://alive2.llvm.org/ce/z/_7dEyJ
binary operations (bitwise): https://alive2.llvm.org/ce/z/cezjVN
vector/aggregate operations: https://alive2.llvm.org/ce/z/BQ7hWz
unary ops: https://alive2.llvm.org/ce/z/yBRs4q
other ops: https://alive2.llvm.org/ce/z/iXbcFD

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D92203