An entity flag Reachable now only applies to E_Label entities. We had an
appropriate guard for setting this flag, but not for clearing.

Cleanup related to detection of uninitialized scalars with GOTO
statements.

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

	* sem_ch5.adb (Analyze_Statements): Only clear Reachable flag on
	proper label entities.