Implement constant folding for DivFOp

Add a constant folder for DivFOp. Analogous to existing folders for
AddFOp, SubFOp, and MulFOp. Matches the behavior of existing LLVM
constant folding (https://github.com/llvm/llvm-project/blob/999f5da6b3088fa4c0bb9d05b358d015ca74c71f/llvm/lib/IR/ConstantFold.cpp#L1432).

Reviewed By: ftynse

Differential Revision: https://reviews.llvm.org/D94939