[MLIR, test] Fix use of undef FileCheck var

MLIR test Dialect/Linalg/transform-patterns.mlir tries to check for the
absence of a sequence of instructions with several CHECK-NOT with one of
those directives using a variable defined in another. However CHECK-NOT
are checked independently so that is using a variable defined in a
pattern that should not occur in the input.

This commit removes the dependency between those CHECK-NOT by replacing
occurences of variables by the regex that were used to define them.

Note to reviewers: please pay attention to whether the remaining
reference to l0 is correct. There was a l0 defined in those CHECK-NOT
and one defined before. I'm not sure what was the intent there.

Reviewed By: nicolasvasilache

Differential Revision: https://reviews.llvm.org/D99957