[llvm] [MIRPrinter] Don't print line break when there is no instructions (NFC) (PR #80147)

[Quentin Dian via llvm-commits]

DianQK wrote:

@jayfoad @arsenm Do you have any suggestions for this PR? I would like to revert it until the end of the backport of LLVM 18. Otherwise, this could make backport difficult for the next few months.

https://github.com/llvm/llvm-project/pull/80147