[mlir][bufferization] Fix op dominance bug in rewrite pattern (#74159)

Fixes a bug in `SplitDeallocWhenNotAliasingAnyOther`. This pattern used
to generate invalid IR (op dominance error). We never noticed this bug
in existing test cases because other patterns and/or foldings were
applied afterwards and those rewrites "fixed up" the IR again. (The bug
is visible when running `mlir-opt -debug`.) Also add additional comments
to the implementation and simplify the code a bit.

Apart from the fixed dominance error, this change is NFC. Without this
change, buffer deallocation tests will fail when running with #74270.