[llvm] [AMDGPU][SplitModule] Fix a couple of issues (PR #117586)

[Fraser Cormack via llvm-commits]

================
@@ -15,19 +15,20 @@
 ; Also check w/o large kernels processing to verify they are indeed handled
 ; differently.
 
-; P0 is empty
----------------
frasercrmck wrote:

I'm not claiming this is the correct fix for the test, but it gets it passing. I haven't updated the comments so those are now stale. @Pierre-vh what would you expect this test to check, with a fixed ratio?

https://github.com/llvm/llvm-project/pull/117586