build

Factor out DecodeStepT from GenerateT into a separate function. #3478

Job	Run time
bazel	8m 45s
macos-latest (make) Release	1m 3s
ubuntu-20.04 (make) Release	1m 53s
ubuntu-latest (make) Release	3m 56s
windows-latest (windows) Release	20m 44s
	36m 21s

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