[mlir][Affine] Fix AffineLoopInvariantCodeMotion

Make sure that memory ops that are defined inside the loop are registered
as such in 'defineOp'. In the test provided, the 'mulf' op was hoisted
outside the loop nest even when its 'affine.load' operand was not.

Reviewed By: bondhugula

Differential Revision: https://reviews.llvm.org/D86982