[IR] Ignore the return value of std::remove_if (NFC)

/Users/jiefu/llvm-project/llvm/lib/IR/Instructions.cpp:166:3: error: ignoring return value of function declared with 'nodiscard' attribute [-Werror,-Wunused-result]
  std::remove_if(const_cast<block_iterator>(block_begin()),
  ^~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1 error generated.