[llvm] [NVPTX] Only run LowerUnreachable when necessary (PR #109868)

[Artem Belevich via llvm-commits]

================
@@ -24,25 +26,30 @@ define void @kernel_func() {
   call void @throw()
 ; CHECK-TRAP-NOT: exit;
 ; CHECK-TRAP: trap;
+
 ; CHECK-NOTRAP-NOT: trap;
 ; CHECK: exit;
+
+; CHECK-BUG-FIXED-NOT: exit;
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Artem-B wrote:

Let's convert the test to use autogenerated checks.
The negative checks give me headaches and are often broken.

https://github.com/llvm/llvm-project/pull/109868