[mlir][linalg][bufferize][NFC] Allow returning arbitrary memrefs
If allowReturnMemref is set to true, arbitrary memrefs may be returned from FuncOps. Also remove allocation hoisting code, which is only partly implemented at the moment.
The purpose of this commit is to untangle bufferize from aliasInfo. (Even with this change, they are not fully untangled yet.)
Differential Revision: https://reviews.llvm.org/D114507