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Speed up synthesis passes when there is nothing to synthesize (#10788)

* Speed up synthesis passes when there is nothing to synthesize

This commit makes a small optimization to the UnitarySynthesis and
HighLevelSynthesis synthesis passes when there are no gates to
synthesize. Previously these passes would iterate over the entire DAG
and check every gate to determine that None needed to be synthesized.
While this is relatively quick for very large circuits this can take
100s of ms. However, this was wasted work because the DAG carries a
dictionary of op names in the circuit so we can determine in constant
time whether any of the operations to synthesize are present up front
and skip the bulk of the iteration. To improve the impact here the
DAGCircuit.count_ops() method is updated to skip the recursion if no
control flow operations are present. Otherwise we'd still be iterating
over the DAG (in a faster manner) which would limit the improvement
from this commit. This isn't the best approach for that as it uses a
hardcoded list of control flow operations which doesn't seem super
general purpose, but as this hard coded set of instructions is
already used elsewhere in the DAGCircuit this isn't making the
situation any worse. If/when we expand recursive objects this can be
revisited (and we'll likely not be able to avoid the iteration in
count ops). With these changes the time these passes take when there are
no operations to synthesize only takes ~5-15 microseconds.

A similar optimization should be considered for UnrollCustomDefinitions
as this takes an even longer amount of time to iterate when there is
nothing to translate. However, it wasn't done as part of this PR as to
check the equivalence library has an entry we need to work with gate
objects (well actually name and number of qubits as that's all that's
used from the object for a lookup) and the name from the op count
dictionary is not sufficient for a lookup. So this will need to be
handled inde... (continued)

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