Plane: autoland: improve performance with range of loiter altitudes #1620

Job	Run time
build (base)	2m 40s
build (clang)	2m 3s
autotest (sitltest-plane-tests1a)	15m 18s
autotest (sitltest-plane-tests1b)	23m 44s
autotest (sitltest-quadplane)	15m 1s
	58m 46s

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