checker: don't cast match branch structs to IError, when the return t… #7211

Job	Run time
tests-sanitize-undefined-clang	1h 0m 11s
tests-sanitize-undefined-gcc	1h 32m 1s
tests-sanitize-address-clang	2h 4m 2s
tests-sanitize-address-gcc	2h 29m 33s
tests-sanitize-address-msvc	3m 59s
tests-sanitize-memory-clang	3h 18m 27s
	10h 28m 13s

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