[llvm] [AMDGPU] Only treat unconditional branches as implicit XCNT drains (PR #219206)

[Jay Foad via llvm-commits]

jayfoad wrote:

But there is still a bug, right? Your fix seemed reasonable, just the description was wrong. It's not that untaken conditional branches do not drain XCNT, it's that the conditional branch got removed by a later pass.

https://github.com/llvm/llvm-project/pull/219206