[mlir] Fix -Wunused-but-set-variable in ModuleTranslation.cpp (NFC)

mlir/lib/Target/LLVMIR/ModuleTranslation.cpp:1050:11:
error: variable 'numConstantsHit' set but not used [-Werror,-Wunused-but-set-variable]
      int numConstantsHit = 0;
          ^
mlir/lib/Target/LLVMIR/ModuleTranslation.cpp:1051:11:
error: variable 'numConstantsErased' set but not used [-Werror,-Wunused-but-set-variable]
      int numConstantsErased = 0;
          ^
2 errors generated.