Typecheck unreachable code #15110

Job	Run time
Test mypyc suite with 32-bit Python	7m 25s
Test suite with py39-ubuntu, mypyc-compiled	21m 19s
Test suite with py39-windows-64	22m 6s
Test suite with py310-ubuntu	13m 58s
Test suite with py311-ubuntu, mypyc-compiled	17m 57s
Test suite with py312-ubuntu, mypyc-compiled	13m 30s
Test suite with py313-ubuntu, mypyc-compiled	21m 57s
mypyc runtime tests with py39-macos	4m 31s
Type check our own code (py39-ubuntu)	2m 3s
Type check our own code (py39-windows-64)	3m 31s
Formatting and code style with Black + ruff	1m 40s
	2h 9m 57s

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