Prevent a test from seeing forbidden numbers in the rustc version
The final CHECK-NOT directive in this test was able to see past the end of the enclosing function, and find the substring `753` or `754` in the git hash in the rustc version number, causing false failures in CI whenever the git hash happens to contain those digits in sequence.
Adding an explicit check for `ret` prevents the CHECK-NOT directive from seeing past the end of the function.
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Manually tested by adding `// CHECK-NOT: rustc` after the existing CHECK-NOT directives, and demonstrating that the new check prevents it from seeing the rustc version string.
f4aa209e20