Fixed a memory leak in the PDLToPDLInterp RootOrderingTest.

RootOrderingTest is a low-level unit test that creates values and uses them as vertices in a directed graph. These vertices were created using `builder.create`, but never freed, due to my insufficient understanding of the MLIR infrastructure.

Reviewed By: mehdi_amini, bondhugula, rriddle

Differential Revision: https://reviews.llvm.org/D114745