Improve BackendConcrete is_true/is_false fast paths #81

Job	Run time
ecosystem / Build	3m 25s
ecosystem / Lint	52s
ecosystem / Test (0)	8m 54s
ecosystem / Test (1)	6m 18s
ecosystem / Test (2)	7m 33s
ecosystem / Test (3)	5m 42s
ecosystem / Test (4)	6m 13s
ecosystem / Test (5)	4m 45s
ecosystem / Test (6)	6m 55s
ecosystem / Test (7)	4m 42s
ecosystem / Test (8)	8m 36s
ecosystem / Test (9)	6m 24s
ecosystem / Publish Unit Tests Results	1s
	1h 10m 20s

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