[possibly-undefined] Fix loop misconceptions #13146

Job	Run time
Test mypyc suite with 32-bit Python	7m 22s
Test suite with Python nightly	13m 59s
Test suite with py38-ubuntu, mypyc-compiled	17m 31s
Test suite with py38-windows-64	20m 54s
Test suite with py39-ubuntu	18m 45s
Test suite with py310-ubuntu	15m 50s
Test suite with py311-ubuntu, mypyc-compiled	15m 29s
Test suite with py312-ubuntu, mypyc-compiled	17m 32s
mypyc runtime tests with py39-macos	5m 38s
mypyc runtime tests with py38-debug-build-ubuntu	4m 31s
Type check our own code (py38-ubuntu)	1m 46s
Type check our own code (py38-windows-64)	2m 8s
Formatting and code style with Black + ruff	35s
	2h 22m 0s

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