Performance: Speed up quantum allocation combining

[taalexander]

CombineQuantumAllocations currently adds every operation in a function to its
greedy rewrite worklist, even though its patterns only apply to quantum
allocations and the operations that form views of them.

This change builds the worklist from those operations instead. Newly created
operations remain eligible for rewriting, and nested candidates are included
because they can refer to an allocation from an enclosing region.

Performance

Each generated kernel started with 16 single-qubit allocations, which the pass
combined into one allocation. The number of gates was much larger than the
number of allocations, which is why avoiding a rewrite-worklist entry for every
gate matters here:

• 16 allocations and 500,000 gates: 1.44 seconds to 0.19 seconds, about 7.5x
  faster.
• 16 allocations and 1,500,000 gates: 4.86 seconds to 0.51 seconds, about 9.5x
  faster.

Peak memory was also lower in both cases.

[github-actions]

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