Add fast path to analyzing special form assignments (#16561) #413

Job	Run time
Test mypyc suite with 32-bit Python	8m 22s
Test suite with py38-ubuntu, mypyc-compiled	18m 38s
Test suite with py38-windows-64	27m 10s
Test suite with py39-ubuntu	19m 21s
Test suite with py310-ubuntu	18m 18s
Test suite with py311-ubuntu, mypyc-compiled	17m 31s
Test suite with py312-ubuntu, mypyc-compiled	21m 17s
mypyc runtime tests with py39-macos	6m 12s
mypyc runtime tests with py38-debug-build-ubuntu	5m 11s
Type check our own code (py38-ubuntu)	3m 24s
Type check our own code (py38-windows-64)	2m 10s
Formatting and code style with Black + ruff	38s
	2h 28m 12s

Provide feedback