Fix: returnShadow size #8851

Job	Run time
Enzyme CI LLVM 15 Debug ubuntu-22.04	4m 0s
Enzyme CI LLVM 16 Debug ubuntu-22.04	3m 35s
Enzyme CI LLVM 16 Release macOS	7m 24s
Enzyme CI LLVM 15 Release ubuntu-22.04	4m 43s
Enzyme CI LLVM 16 Release ubuntu-22.04	4m 29s
Enzyme CI LLVM 16 Debug macOS	6m 50s
Enzyme CI LLVM 16 Release macOS XCode	4m 2s
Enzyme CI LLVM 15 Release macOS	5m 6s
Enzyme CI LLVM 15 Debug macOS	7m 22s
	47m 31s

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