[possibly-undefined] Fix loop misconceptions #13123

Job	Run time
Test mypyc suite with 32-bit Python	6m 41s
Test suite with Python nightly	14m 4s
Test suite with py38-ubuntu, mypyc-compiled	18m 7s
Test suite with py38-windows-64	20m 19s
Test suite with py39-ubuntu	16m 48s
Test suite with py310-ubuntu	15m 45s
Test suite with py311-ubuntu, mypyc-compiled	15m 45s
Test suite with py312-ubuntu, mypyc-compiled	17m 50s
mypyc runtime tests with py39-macos	4m 25s
mypyc runtime tests with py38-debug-build-ubuntu	4m 22s
Type check our own code (py38-ubuntu)	1m 48s
Type check our own code (py38-windows-64)	2m 9s
Formatting and code style with Black + ruff	36s
	2h 18m 39s

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