merge edges in CFG visualization #3423

Job	Run time
macOS / release / homebrew-clang	4m 39s
macOS / debug / homebrew-clang	5m 1s
Windows / release / msvc	27m 39s
Windows / release / clang-cl	22m 41s
macOS / release / system-clang	4m 19s
Windows / release / llvm	21m 58s
macOS / debug / system-clang	4m 27s
Windows / debug / msvc	23m 2s
Windows / debug / clang-cl	21m 40s
Windows / debug / llvm	25m 21s
	2h 40m 47s

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