[MLIR] Remove hardcoded usage of alloc/dealloc; use memory effects

Remove hardcoded usage of memref.alloc/dealloc ops in affine utils; use memory
effects instead. This is NFC for existing test cases, but strictly more
general/powerful in terms of functionality: it allows other allocation and
freeing ops (like gpu.alloc/dealloc) to work in conjunction with affine ops and
utilities.

Differential Revision: https://reviews.llvm.org/D132011