[mlir] Fix -Wsign-compare in MemRefOps.cpp (NFC)

llvm-project/mlir/lib/Dialect/MemRef/IR/MemRefOps.cpp:2763:23:
error: comparison of integers of different signs: 'int64_t' (aka 'long') and 'size_type' (aka 'unsigned long') [-Werror,-Wsign-compare]
  assert(t1.getRank() == droppedDims.size() && "incorrect number of bits");
         ~~~~~~~~~~~~ ^  ~~~~~~~~~~~~~~~~~~

llvm-project/mlir/lib/Dialect/MemRef/IR/MemRefOps.cpp:2764:38:
error: comparison of integers of different signs: 'int64_t' (aka 'long') and 'size_type' (aka 'unsigned long') [-Werror,-Wsign-compare]
  assert(t1.getRank() - t2.getRank() == droppedDims.count() &&
         ~~~~~~~~~~~~~~~~~~~~~~~~~~~ ^  ~~~~~~~~~~~~~~~~~~~