[MLIR] Fix incorrect assertion in memref replacement utility

The assertion had an unchecked use of getDefiningOp. The condition
itself is stale and not needed. This is a trivial/obvious fix - the
buggy path wasn't exercised since the current passes/users of this
utility always passed the result of an op as an "extra index".

Differential Revision: https://reviews.llvm.org/D138191