ada: Crash on call to a dispatching op with if_expr and tag-indeterminate calls

ada: Crash on call to a dispatching op with if_expr and tag-indeterminate calls

The compiler can crash with a Storage_Error for a failed precondition
when compiling a call to a dispatching subprogram where an actual for a
controlling formal is given by an if_expression whose dependent expressions
are tag-indeterminate calls.  This problem showed up on a build of Alire
(on the compilation of alire-roots.adb) done during a build_gnat_world
mailserver (as well as having been noticed and reported separately by
another engineering team).

The code in Sem_Disp.Check_Dispatching_Call for checking nondispatching
procedure calls for actuals that are calls to abstract tag-indeterminate
functions did not account for conditional expressions, and attempted to
retrieve the (nonexistent) Expression field from an N_If_Expression node,
failing the precondition for Expression.  It was discovered that none of
the code checking for illegal calls to abstract tag-indeterminate functions
in procedure calls is needed, and the whole related "elsif" part is removed
by this change.  (Note that there is separate checking done separately
within Check_Dispatching_Call that will catch nondispatching calls to
abstract functions.)

gcc/ada/ChangeLog:

	* sem_disp.adb (Check_Dispatching_Call): Remove "elsif" that does error
	checking for abstract tag-indeterminate calls (seems to be no longer
	needed).