Do not make a local copy of large aggregate

Do not make a local copy of large aggregate

This prevents gigi from making a local copy of large aggregates.

	* gcc-interface/trans.c (lvalue_required_p) <N_Selected_Component>:
	Merge with N_Slice.
	<N_Allocator>: Move to...
	(lvalue_for_aggregate_p): ...here.  New function.
	(Identifier_to_gnu): For an identifier with aggregate type, also
	call lvalue_for_aggregate_p if lvalue_required_p returned false
	before substituting the identifier with the constant.