DCE enhancement to delete dead vptr assignment

[Xinliang David Li]

This is related to PR34949. It makes use C++ language semantics.
Currently it handles empty dtors or their inline instances into
deleting dtors or inline instances into callers followed by delete
call.  It will extend automatically to handle other cases (e.g. dtor
with printf statement etc) when more precise alias/mod-ref or builtin
function side effect information is available.

bootstrap and regression on x86-64/linux

Ok for mainline?

Thanks,

David
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2010-04-15  Xinliang David Li  <davidxl@google.com>

	* tree-ssa-dce.c (find_base_ptr): New function.
	(is_killed_by_delete): New function.
	(mark_operand_necessary): Handle vptr field assignment.
	(mark_stmt_if_obviously_necessary): Handle vptr field assignment.
	* langhooks.c (lhs_do_nothing_t_return_false): New function.
	* langhooks.h: New lang hooks.
	* tree-flow.h: New interfaces.
	* gimple.c (is_vptr_init): New function.
	(is_vptr_access): New function.
	(is_global_delete_call): New function.
	(points_to_vbtl): New function.
	* cp/cp-lang.c (cp_is_decl_dtor_p): New function.
	(cp_is_vptr_field_p): New function.
	(cp_is_polymorphic_type_p): New function.
	(cp_is_global_delete_fndecl_p): New function.
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2010-04-15  Xinliang David Li  <davidxl@google.com>

	* g++.dg/tree-ssa/vptr_init_dse.C: New test.
	* g++.dg/tree-ssa/vptr_init_dse2.C: New test.
	* g++.dg/tree-ssa/vptr_init_dse3.C: New test.
	* g++.dg/tree-ssa/vptr_init_dse4.C: New test.