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

[Pierre van Houtryve via llvm-commits]

Pierre-vh wrote:

> Is it because we have some output/print are via stdout and some via stderr?

Yes, and the test relies on this ordering for its output.

I think the right move in the end is to move all those `CHECK: error` to check for actual line numbers using `:[[@LINE+1]]:{{[0-9]+}}:` which is the usual pattern I see for this kind of stuff.

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