Add a folder for division by one.

- Adds a folder for integer division by one with the `divi_signed` and `divi_unsigned` ops.
- Creates tests for scalar and tensor versions of these ops.
- Modifies the test in `parallel-loop-collapsing.mlir` so that it doesn't assume division by one will be in the output.

Differential Revision: https://reviews.llvm.org/D78518