[MLIR, test] Fix use of undef FileCheck var

MLIR test Dialect/Linalg/tile-indexed-generic.mlir has a CHECK-NOT
directive referring to a variable only defined in a CHECK directive with
a different prefix, and thus undefined in the CHECK-NOT.

This commit removes the variable reference altogether to error on any
content it might have.

Reviewed By: pifon2a

Differential Revision: https://reviews.llvm.org/D99956