[mlir][scf][bufferize][NFC] Lookup buffer using helper function

Lookup iter_arg buffers using `lookupBuffer` instead of always creating a new `ToMemrefOp`. Also cast all yielded buffers (if necessary), regardless of whether they are an equivalent buffer or a new allocation.

Note: This should have been part of D123369.

Differential Revision: https://reviews.llvm.org/D123383