Fix #14371: FP redundantAssignment for nested union members #20458

Job	Run time
build_cmake (ubuntu:25.10, true, true)	3m 27s
build_cmake (ubuntu:24.04, true, true)	4m 2s
build_make (ubuntu:25.10)	4m 29s
build_make (ubuntu:24.04)	3m 30s
build_cmake (alpine:3.23, false, false)	3m 36s
build_make (alpine:3.23)	6m 4s
	25m 8s

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