abg-ir.cc: Refactor operator== methods with helper function

abg-ir.cc: Refactor operator== methods with helper function

Many of the operator== definitions in this source file follow the same
pattern:

- First, canonical comparison is attempted if canonical types are
  present.
- Otherwise, the comparison is performed structurally using the
  'equals' function.

This commit refactors the common logic into a templated helper
function named "try_canonical_compare".

There are no behavioural changes.

	* src/abg-ir.cc (try_canonical_compare): New template function.
	(type_decl::operator==): Use it here.
	(scope_type_decl::operator==): Likewise.
	(qualified_type_def::operator==): Likewise.
	(pointer_type_def::operator==): Likewise.
	(reference_type_def::operator==): Likewise.
	(array_type_def::subrange_type::operator==): Likewise.
	(array_type_def::operator==): Likewise.
	(enum_type_decl::operator==): Likewise.
	(typedef_decl::operator==): Likewise.
	(function_type::operator==): Likewise.
	(class_or_union::operator==): Likewise.
	(class_decl::operator==): Likewise.
	(union_decl::operator==): Likewise.

Signed-off-by: Giuliano Procida <gprocida@google.com>
Signed-off-by: Dodji Seketeli <dodji@redhat.com>