[possibly-undefined] Fix loop misconceptions #13141

Job	Run time
Test mypyc suite with 32-bit Python	4m 28s
Test suite with Python nightly	4m 28s
Test suite with py38-ubuntu, mypyc-compiled	4m 28s
Test suite with py38-windows-64	4m 4s
Test suite with py39-ubuntu	4m 34s
Test suite with py310-ubuntu	4m 30s
Test suite with py311-ubuntu, mypyc-compiled	4m 30s
Test suite with py312-ubuntu, mypyc-compiled	4m 29s
mypyc runtime tests with py39-macos	4m 30s
mypyc runtime tests with py38-debug-build-ubuntu	4m 29s
Type check our own code (py38-ubuntu)	1m 47s
Type check our own code (py38-windows-64)	2m 18s
Formatting and code style with Black + ruff	38s
	49m 13s

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