[TableGen] Allocate `Operator` object on heap in `RecordOperatorMap`
Iterators for a `llvm::DenseMap` can be invalidated when an insertion occurs.
In Pattern's `collectBoundArguments()`, we recursively handle all nested DAG
nodes and grow the the `RecordOperatorMap`, while retaining a reference.
This can cause the reference to be invalid and the program to behave randomly.
Allocate each `Operator` object specifically to solve this issue.
Also, `llvm::DenseMap` is a great way to map pointers to pointers, or map
other small types to each other. This avoids placing the `Operator` object
directly into the map.
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PiperOrigin-RevId:
243281486