[llvm] [NFC]Add assert to avoid possibly deref nullptr (PR #65564)

[Adrian Prantl via llvm-commits]

adrian-prantl wrote:

@XinWang10 This patch doesn't fix the issue that you discovered. Most vendors ship LLVM with assertions disable for performance reasons. Can you fix this properly by either making the code that depends on MMI conditional or by making it impossible to have a null MMI member in AsmPrinter?

https://github.com/llvm/llvm-project/pull/65564