[MLIR][MemRef] Avoid returning ReallocOp from findDealloc

This diff makes findDealloc return nullopt if the list of users contains "realloc".
Fixes https://github.com/llvm/llvm-project/issues/60726
(In particular, in SimplifyClones (uses findDealloc) treating "realloc" as "dealloc"
breaks the assumption that "dealloc" has no users)

Test plan: ninja check-mlir check-all

Differential revision: https://reviews.llvm.org/D154892