[possibly-undefined] Fix loop misconceptions #13139

Job	Run time
Test mypyc suite with 32-bit Python	4m 22s
Test suite with Python nightly	4m 20s
Test suite with py38-ubuntu, mypyc-compiled	4m 19s
Test suite with py38-windows-64	4m 10s
Test suite with py39-ubuntu	4m 20s
Test suite with py310-ubuntu	4m 20s
Test suite with py311-ubuntu, mypyc-compiled	4m 18s
Test suite with py312-ubuntu, mypyc-compiled	4m 20s
mypyc runtime tests with py39-macos	4m 18s
mypyc runtime tests with py38-debug-build-ubuntu	4m 19s
Type check our own code (py38-ubuntu)	1m 51s
Type check our own code (py38-windows-64)	2m 39s
Formatting and code style with Black + ruff	39s
	48m 15s

Provide feedback