Test runner silently ignores --check-prefixes (plural), so those RUN lines check nothing

The compile-test runner parses only --check-prefix= (singular). A RUN line using --check-prefixes=A,B falls back to the default prefix, and if the file has no // CHECK: lines that RUN line contributes zero checks and passes.

tests/backend/pass/cpu_fusion_overhead.vx has three RUN lines:

// RUN: vxc %s --emit-mlir -O0 | FileCheck %s --check-prefixes=MLIR,MLIR-O0
// RUN: vxc %s --emit-mlir -O3 | FileCheck %s --check-prefixes=MLIR,MLIR-O3
// RUN: vxc %s --emit-llvm -O3 | FileCheck %s --check-prefix=LLVM

The first two check nothing at all. npu_fusion_overhead.vx is the same shape.

How it surfaced

The runner understands only CHECK: and CHECK-NOT: as ordered substrings; anything else contributes no checks. There is a guard that refuses a file using the other directives, since passing-without-checking is indistinguishable from passing for the right reason -- but the guard only looked for CHECK-*. A file that declares its own prefix and then writes MYPREFIX-SAME: walked straight past it.

That guard now derives the prefixes from the file's own RUN lines, and immediately found // LLVM-LABEL: module { in both files above. Converting it to an enforceable // LLVM: module { then failed -- the output that RUN line produces does not contain that text. So the directive had been asserting something untrue for as long as it has existed, invisibly. Removed, with a note in place so it is not reinstated as a harmless-looking label.

--check-prefixes is the same class of hole one level up, and is left as-is here because fixing it will expose whatever those two RUN lines were meant to assert, which is a separate question from the change that found it.

Worth deciding

Either support the directives (real FileCheck semantics), or make the runner reject what it cannot enforce -- including plural prefixes and any prefix with no matching check lines. The second is cheaper and is the same principle already applied to CHECK-DAG/NEXT/SAME: a check the runner cannot perform should fail the file rather than be skipped.

A RUN line that produces no checks is the specific thing to catch. That is a one-line condition and it would have caught all four cases here.

Start in the compile-test runner's handling of FileCheck RUN lines and its existing guard for unsupported directives. Reproduce the cases in tests/backend/pass/cpu_fusion_overhead.vx and npu_fusion_overhead.vx, then add coverage for plural prefixes and RUN lines that produce no checks. Done means unsupported or unchecked RUN lines are rejected, or plural-prefix semantics are fully enforced.