[possibly-undefined] Fix loop misconceptions #13147

Job	Run time
Test mypyc suite with 32-bit Python	6m 54s
Test suite with py38-ubuntu, mypyc-compiled	17m 13s
Test suite with Python nightly	14m 2s
Test suite with py38-windows-64	19m 54s
Test suite with py39-ubuntu	18m 35s
Test suite with py310-ubuntu	15m 55s
Test suite with py311-ubuntu, mypyc-compiled	16m 11s
Test suite with py312-ubuntu, mypyc-compiled	18m 31s
mypyc runtime tests with py39-macos	4m 32s
mypyc runtime tests with py38-debug-build-ubuntu	4m 34s
Type check our own code (py38-ubuntu)	1m 47s
Type check our own code (py38-windows-64)	2m 28s
Formatting and code style with Black + ruff	33s
	2h 21m 9s

Provide feedback