[llvm] [AMDGPU] Model waterfall loop EXEC update as a terminator (PR #219519)

[Matt Arsenault via llvm-commits]

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+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py UTC_ARGS: --version 6
+# RUN: llc -mtriple=amdgcn-amd-amdhsa -mcpu=gfx1010 -mattr=+wavefrontsize32 --run-pass=si-optimize-exec-masking -o - %s | FileCheck %s
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arsenm wrote:

```suggestion
# RUN: llc -mtriple=amdgpu10.10-amd-amdhsa -run-pass=si-optimize-exec-masking -o - %s | FileCheck %s
```

https://github.com/llvm/llvm-project/pull/219519