[llvm] [NFC][AMDGPU] Do not flush after printing every instruction (PR #95237)

[Jay Foad via llvm-commits]

jayfoad wrote:

I would suggest changing the test to have two RUN lines, one to test stdout and one to test stderr. It should not be relying on `-NEXT:` checks where one line is stderr and the next is stdout!

Or, only flush stdout just before emitting an error message to stderr???

https://github.com/llvm/llvm-project/pull/95237