[MLIR, test] Fix use of undef FileCheck var

MLIR test Transforms/canonicalize.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.

Reviewed By: pifon2a

Differential Revision: https://reviews.llvm.org/D99958