[mlir] Fix non-const lvalue reference to type 'uint64_t' cannot bind to type 'size_t' error (NFC)

/Users/jiefu/llvm-project/mlir/lib/Bytecode/Reader/BytecodeReader.cpp:1007:39: error: non-const lvalue reference to type 'uint64_t' (aka 'unsigned long long') cannot bind to a value of unrelated type 'size_t' (aka 'unsigned long')
    if (failed(propReader.parseVarInt(count)))
                                      ^~~~~
/Users/jiefu/llvm-project/mlir/lib/Bytecode/Reader/BytecodeReader.cpp:191:39: note: passing argument to parameter 'result' here
  LogicalResult parseVarInt(uint64_t &result) {
                                      ^
/Users/jiefu/llvm-project/mlir/lib/Bytecode/Reader/BytecodeReader.cpp:1020:44: error: non-const lvalue reference to type 'uint64_t' (aka 'unsigned long long') cannot bind to a value of unrelated type 'size_t' (aka 'unsigned long')
      if (failed(offsetsReader.parseVarInt(dataSize)) ||
                                           ^~~~~~~~
/Users/jiefu/llvm-project/mlir/lib/Bytecode/Reader/BytecodeReader.cpp:191:39: note: passing argument to parameter 'result' here
  LogicalResult parseVarInt(uint64_t &result) {
                                      ^
2 errors generated.