Don't flag `redefined-while-unused` in if branches #23275

Job	Run time
cargo fmt	13s
python package	1m 10s
Determine changes	7s
mkdocs	2m 4s
pre-commit	25s
cargo test (linux)	3m 49s
cargo fuzz	4m 11s
cargo test (wasm)	1m 17s
cargo clippy	1m 54s
cargo test (windows)	5m 38s
test scripts	54s
benchmarks	6m 10s
cargo udeps	1m 52s
formatter instabilities and black similarity	0s
test ruff-lsp	16s
ecosystem	8m 29s
	38m 29s

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