d: Refactor use of built-in memcmp/memcpy/memset into helper functions.

d: Refactor use of built-in memcmp/memcpy/memset into helper functions.

Generating calls to memset, memcpy, and memcmp is frequent enough that
it becomes beneficial to put them into their own routine.  All parts of
the front-end have been updated to call the new helper functions instead
of doing it themselves.

gcc/d/ChangeLog:

	* d-codegen.cc (build_memcmp_call): New function.
	(build_memcpy_call): New function.
	(build_memset_call): New function.
	(build_float_identity): Call build_memcmp_call.
	(lower_struct_comparison): Likewise.
	(build_struct_comparison): Likewise.
	* d-tree.h (build_memcmp_call): Declare.
	(build_memcpy_call): Declare.
	(build_memset_call): Declare.
	* expr.cc (ExprVisitor::visit (EqualExp *)): Call build_memcmp_call.
	(ExprVisitor::visit (AssignExp *)): Call build_memset_call.
	(ExprVisitor::visit (ArrayLiteralExp *)): Call build_memcpy_call.
	(ExprVisitor::visit (StructLiteralExp *)): Call build_memset_call.