[mypyc] Don't crash on unreachable statements #11379

Job	Run time
Test suite with py38-ubuntu, mypyc-compiled	25m 34s
Test mypyc suite with 32-bit Python	11m 14s
Test suite with py38-windows-64	34m 33s
Test suite with py39-ubuntu	23m 54s
Test suite with py310-ubuntu	32m 4s
Test suite with py311-ubuntu, mypyc-compiled	24m 0s
Test suite with py312-ubuntu, mypyc-compiled	24m 52s
mypyc runtime tests with py38-macos	9m 15s
mypyc runtime tests with py38-debug-build-ubuntu	7m 21s
Type check our own code (py38-ubuntu)	2m 48s
Type check our own code (py38-windows-64)	3m 12s
Formatting and code style with Black + ruff	1m 17s
	3h 20m 4s

Provide feedback