Fix #14371: FP redundantAssignment for nested union members #20451

Job	Run time
build_cmake (ubuntu:24.04, true, true)	5m 43s
build_make (ubuntu:25.10)	5m 1s
build_make (ubuntu:24.04)	4m 41s
build_make (alpine:3.23)	6m 13s
build_cmake (ubuntu:25.10, true, true)	5m 33s
build_cmake (alpine:3.23, false, false)	4m 29s
	31m 40s

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