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.
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.
Adding an explicit check for `ret` prevents the CHECK-NOT directive from seeing
past the end of the function.
