[MLIR] Fix compiler warnings (NFC)

In `TestTensorTransforms.cpp` `replaced` is nullptr I assumed the intent
was to emit the error for the `rootOp`.

In `TransformInterfaces.cpp` there were some uninitialized variables.

In `NVGPUTransformOps.cpp` `matmulOp` was never used.

Reviewed By: ftynse

Differential Revision: https://reviews.llvm.org/D154439